.\" Automatically generated by Pod::Man 4.11 (Pod::Simple 3.35) .\" .\" Standard preamble: .\" ======================================================================== .de Sp \" Vertical space (when we can't use .PP) .if t .sp .5v .if n .sp .. .de Vb \" Begin verbatim text .ft CW .nf .ne \\$1 .. .de Ve \" End verbatim text .ft R .fi .. .\" Set up some character translations and predefined strings. \*(-- will .\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left .\" double quote, and \*(R" will give a right double quote. \*(C+ will .\" give a nicer C++. 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No user-serviceable parts. . \" fudge factors for nroff and troff .if n \{\ . ds #H 0 . ds #V .8m . ds #F .3m . ds #[ \f1 . ds #] \fP .\} .if t \{\ . ds #H ((1u-(\\\\n(.fu%2u))*.13m) . ds #V .6m . ds #F 0 . ds #[ \& . ds #] \& .\} . \" simple accents for nroff and troff .if n \{\ . ds ' \& . ds ` \& . ds ^ \& . ds , \& . ds ~ ~ . ds / .\} .if t \{\ . ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u" . ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u' . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u' . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u' . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u' . ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u' .\} . \" troff and (daisy-wheel) nroff accents .ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V' .ds 8 \h'\*(#H'\(*b\h'-\*(#H' .ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#] .ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H' .ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u' .ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#] .ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#] .ds ae a\h'-(\w'a'u*4/10)'e .ds Ae A\h'-(\w'A'u*4/10)'E . \" corrections for vroff .if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u' .if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u' . \" for low resolution devices (crt and lpr) .if \n(.H>23 .if \n(.V>19 \ \{\ . ds : e . ds 8 ss . ds o a . ds d- d\h'-1'\(ga . ds D- D\h'-1'\(hy . ds th \o'bp' . ds Th \o'LP' . ds ae ae . ds Ae AE .\} .rm #[ #] #H #V #F C .\" ======================================================================== .\" .IX Title "RRDTUTORIAL 1" .TH RRDTUTORIAL 1 "2024-07-29" "1.9.0" "rrdtool" .\" For nroff, turn off justification. Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" rrdtutorial \- Alex van den Bogaerdt's RRDtool tutorial .SH "DESCRIPTION" .IX Header "DESCRIPTION" RRDtool is written by Tobias Oetiker with contributions from many people all around the world. This document is written by Alex van den Bogaerdt to help you understand what RRDtool is and what it can do for you. .PP The documentation provided with RRDtool can be too technical for some people. This tutorial is here to help you understand the basics of RRDtool. It should prepare you to read the documentation yourself. It also explains the general things about statistics with a focus on networking. .SH "TUTORIAL" .IX Header "TUTORIAL" .SS "Important" .IX Subsection "Important" Please don't skip ahead in this document! The first part of this document explains the basics and may be boring. But if you don't understand the basics, the examples will not be as meaningful to you. .PP Sometimes things change. This example used to provide numbers like \&\*(L"0.04\*(R" instead of \*(L"4.00000e\-02\*(R". Those are really the same numbers, just written down differently. Don't be alarmed if a future version of rrdtool displays a slightly different form of output. The examples in this document are correct for version 1.2.0 of RRDtool. .PP Also, sometimes bugs do occur. They may also influence the outcome of the examples. Example speed4.png was suffering from this (the handling of unknown data in an if-statement was wrong). Normal data will be just fine (a bug in rrdtool wouldn't last long) but special cases like NaN, \s-1INF\s0 and so on may last a bit longer. Try another version if you can, or just live with it. .PP I fixed the speed4.png example (and added a note). There may be other examples which suffer from the same or a similar bug. Try to fix it yourself, which is a great exercise. But please do not submit your result as a fix to the source of this document. Discuss it on the user's list, or write to me. .SS "What is RRDtool?" .IX Subsection "What is RRDtool?" RRDtool refers to Round Robin Database tool. Round robin is a technique that works with a fixed amount of data, and a pointer to the current element. Think of a circle with some dots plotted on the edge. These dots are the places where data can be stored. Draw an arrow from the center of the circle to one of the dots; this is the pointer. When the current data is read or written, the pointer moves to the next element. As we are on a circle there is neither a beginning nor an end, you can go on and on and on. After a while, all the available places will be used and the process automatically reuses old locations. This way, the dataset will not grow in size and therefore requires no maintenance. RRDtool works with Round Robin Databases (RRDs). It stores and retrieves data from them. .SS "What data can be put into an \s-1RRD\s0?" .IX Subsection "What data can be put into an RRD?" You name it, it will probably fit as long as it is some sort of time-series data. This means you have to be able to measure some value at several points in time and provide this information to RRDtool. If you can do this, RRDtool will be able to store it. The values must be numerical but don't have to be integers, as is the case with \s-1MRTG\s0 (the next section will give more details on this more specialized application). .PP Many examples below talk about \s-1SNMP\s0 which is an acronym for Simple Network Management Protocol. \*(L"Simple\*(R" refers to the protocol. It does not mean it is simple to manage or monitor a network. After working your way through this document, you should know enough to be able to understand what people are talking about. For now, just realize that \&\s-1SNMP\s0 can be used to query devices for the values of counters they keep. It is the value from those counters that we want to store in the \s-1RRD.\s0 .SS "What can I do with this tool?" .IX Subsection "What can I do with this tool?" RRDtool originated from \s-1MRTG\s0 (Multi Router Traffic Grapher). \s-1MRTG\s0 started as a tiny little script for graphing the use of a university's connection to the Internet. \s-1MRTG\s0 was later (ab\-)used as a tool for graphing other data sources including temperature, speed, voltage, number of printouts and the like. .PP Most likely you will start to use RRDtool to store and process data collected via \s-1SNMP.\s0 The data will most likely be bytes (or bits) transferred from and to a network or a computer. But it can also be used to display tidal waves, solar radiation, power consumption, number of visitors at an exhibition, noise levels near an airport, temperature on your favorite holiday location, temperature in the fridge and whatever your imagination can come up with. .PP You only need a sensor to measure the data and be able to feed the numbers into RRDtool. RRDtool then lets you create a database, store data in it, retrieve that data and create graphs in \s-1PNG\s0 format for display on a web browser. Those \s-1PNG\s0 images are dependent on the data you collected and could be, for instance, an overview of the average network usage, or the peaks that occurred. .SS "What if I still have problems after reading this document?" .IX Subsection "What if I still have problems after reading this document?" First of all: read it again! You may have missed something. If you are unable to compile the sources and you have a fairly common \&\s-1OS,\s0 it will probably not be the fault of RRDtool. There may be pre-compiled versions around on the Internet. If they come from trusted sources, get one of those. .PP If on the other hand the program works but does not give you the expected results, it will be a problem with configuring it. Review your configuration and compare it with the examples that follow. .PP There is a mailing list and an archive of it. Read the list for a few weeks and search the archive. It is considered rude to just ask a question without searching the archives: your problem may already have been solved for somebody else! This is true for most, if not all, mailing lists and not only for this particular one. Look in the documentation that came with RRDtool for the location and usage of the list. .PP I suggest you take a moment to subscribe to the mailing list right now by sending an email to with a subject of \*(L"subscribe\*(R". If you ever want to leave this list, just write an email to the same address but now with a subject of \*(L"unsubscribe\*(R". .SS "How will you help me?" .IX Subsection "How will you help me?" By giving you some detailed descriptions with detailed examples. I assume that following the instructions in the order presented will give you enough knowledge of RRDtool to experiment for yourself. If it doesn't work the first time, don't give up. Reread the stuff that you did understand, you may have missed something. .PP By following the examples you get some hands-on experience and, even more important, some background information of how it works. .PP You will need to know something about hexadecimal numbers. If you don't, start with reading bin_dec_hex before you continue here. .SS "Your first Round Robin Database" .IX Subsection "Your first Round Robin Database" In my opinion the best way to learn something is to actually do it. Why not start right now? We will create a database, put some values in it and extract this data again. Your output should be the same as the output that is included in this document. .PP We will start with some easy stuff and compare a car with a router, or compare kilometers (miles if you wish) with bits and bytes. It's all the same: some number over some time. .PP Assume we have a device that transfers bytes to and from the Internet. This device keeps a counter that starts at zero when it is turned on, increasing with every byte that is transferred. This counter will probably have a maximum value. If this value is reached and an extra byte is counted, the counter starts over at zero. This is the same as many counters in the world such as the mileage counter in a car. .PP Most discussions about networking talk about bits per second so let's get used to that right away. Assume a byte is eight bits and start to think in bits not bytes. The counter, however, still counts bytes! In the \s-1SNMP\s0 world most of the counters are 32 bits. That means they are counting from 0 to 4294967295. We will use these values in the examples. The device, when asked, returns the current value of the counter. We know the time that has passes since we last asked so we now know how many bytes have been transferred ***on average*** per second. This is not very hard to calculate. First in words, then in calculations: .IP "1." 3 Take the current counter, subtract the previous value from it. .IP "2." 3 Do the same with the current time and the previous time (in seconds). .IP "3." 3 Divide the outcome of (1) by the outcome of (2), the result is the amount of bytes per second. Multiply by eight to get the number of bits per second (bps). .PP .Vb 1 \& bps = (counter_now \- counter_before) / (time_now \- time_before) * 8 .Ve .PP For some people it may help to translate this to an automobile example. Do not try this example, and if you do, don't blame me for the results! .PP People who are not used to think in kilometers per hour can translate most into miles per hour by dividing km by 1.6 (close enough). I will use the following abbreviations: .PP .Vb 6 \& m: meter \& km: kilometer (= 1000 meters). \& h: hour \& s: second \& km/h: kilometers per hour \& m/s: meters per second .Ve .PP You are driving a car. At 12:05 you read the counter in the dashboard and it tells you that the car has moved 12345 km until that moment. At 12:10 you look again, it reads 12357 km. This means you have traveled 12 km in five minutes. A scientist would translate that into meters per second and this makes a nice comparison toward the problem of (bytes per five minutes) versus (bits per second). .PP We traveled 12 kilometers which is 12000 meters. We did that in five minutes or 300 seconds. Our speed is 12000m / 300s or 40 m/s. .PP We could also calculate the speed in km/h: 12 times 5 minutes is an hour, so we have to multiply 12 km by 12 to get 144 km/h. For our native English speaking friends: that's 90 mph so don't try this example at home or where I live :) .PP Remember: these numbers are averages only. There is no way to figure out from the numbers, if you drove at a constant speed. There is an example later on in this tutorial that explains this. .PP I hope you understand that there is no difference in calculating m/s or bps; only the way we collect the data is different. Even the k from kilo is the same as in networking terms k also means 1000. .PP We will now create a database where we can keep all these interesting numbers. The method used to start the program may differ slightly from \&\s-1OS\s0 to \s-1OS,\s0 but I assume you can figure it out if it works different on yours. Make sure you do not overwrite any file on your system when executing the following command and type the whole line as one long line (I had to split it for readability) and skip all of the '\e' characters. .PP .Vb 5 \& rrdtool create test.rrd \e \& \-\-start 920804400 \e \& DS:speed:COUNTER:600:U:U \e \& RRA:AVERAGE:0.5:1:24 \e \& RRA:AVERAGE:0.5:6:10 .Ve .PP (So enter: \f(CW\*(C`rrdtool create test.rrd \-\-start 920804400 DS ...\*(C'\fR) .SS "What has been created?" .IX Subsection "What has been created?" We created the round robin database called test (test.rrd) which starts at noon the day I started writing this document, 7th of March, 1999 (this date translates to 920804400 seconds as explained below). Our database holds one data source (\s-1DS\s0) named \*(L"speed\*(R" that represents a counter. This counter is read every five minutes (this is the default therefore you don't have to put \f(CW\*(C`\-\-step=300\*(C'\fR). In the same database two round robin archives (RRAs) are kept, one averages the data every time it is read (i.e., there's nothing to average) and keeps 24 samples (24 times 5 minutes is 2 hours). The other averages 6 values (half hour) and contains 10 such averages (e.g. 5 hours). .PP RRDtool works with special time stamps coming from the \s-1UNIX\s0 world. This time stamp is the number of seconds that passed since January 1st 1970 \s-1UTC.\s0 The time stamp value is translated into local time and it will therefore look different for different time zones. .PP Chances are that you are not in the same part of the world as I am. This means your time zone is different. In all examples where I talk about time, the hours may be wrong for you. This has little effect on the results of the examples, just correct the hours while reading. As an example: where I will see \*(L"12:05\*(R" the \s-1UK\s0 folks will see \*(L"11:05\*(R". .PP We now have to fill our database with some numbers. We'll pretend to have read the following numbers: .PP .Vb 10 \& 12:05 12345 km \& 12:10 12357 km \& 12:15 12363 km \& 12:20 12363 km \& 12:25 12363 km \& 12:30 12373 km \& 12:35 12383 km \& 12:40 12393 km \& 12:45 12399 km \& 12:50 12405 km \& 12:55 12411 km \& 13:00 12415 km \& 13:05 12420 km \& 13:10 12422 km \& 13:15 12423 km .Ve .PP We fill the database as follows: .PP .Vb 5 \& rrdtool update test.rrd 920804700:12345 920805000:12357 920805300:12363 \& rrdtool update test.rrd 920805600:12363 920805900:12363 920806200:12373 \& rrdtool update test.rrd 920806500:12383 920806800:12393 920807100:12399 \& rrdtool update test.rrd 920807400:12405 920807700:12411 920808000:12415 \& rrdtool update test.rrd 920808300:12420 920808600:12422 920808900:12423 .Ve .PP This reads: update our test database with the following numbers .PP .Vb 2 \& time 920804700, value 12345 \& time 920805000, value 12357 .Ve .PP etcetera. .PP As you can see, it is possible to feed more than one value into the database in one command. I had to stop at three for readability but the real maximum per line is \s-1OS\s0 dependent. .PP We can now retrieve the data from our database using \*(L"rrdtool fetch\*(R": .PP .Vb 1 \& rrdtool fetch test.rrd AVERAGE \-\-start 920804400 \-\-end 920809200 .Ve .PP It should return the following output: .PP .Vb 1 \& speed \& \& 920804700: nan \& 920805000: 4.0000000000e\-02 \& 920805300: 2.0000000000e\-02 \& 920805600: 0.0000000000e+00 \& 920805900: 0.0000000000e+00 \& 920806200: 3.3333333333e\-02 \& 920806500: 3.3333333333e\-02 \& 920806800: 3.3333333333e\-02 \& 920807100: 2.0000000000e\-02 \& 920807400: 2.0000000000e\-02 \& 920807700: 2.0000000000e\-02 \& 920808000: 1.3333333333e\-02 \& 920808300: 1.6666666667e\-02 \& 920808600: 6.6666666667e\-03 \& 920808900: 3.3333333333e\-03 \& 920809200: nan \& 920809500: nan .Ve .PP Note that you might get more rows than you expect. The reason for this is that you ask for a time range that ends on 920809200. The number that is written behind 920809200: in the list above covers the time range from 920808900 to 920809200, \s-1EXCLUDING 920809200.\s0 Hence to be on the sure side, you receive the entry from 920809200 to 920809500 as well since it \s-1INCLUDES 920809200.\s0 You may also see \*(L"NaN\*(R" instead of \*(L"nan\*(R" this is \s-1OS\s0 dependent. \&\*(L"NaN\*(R" stands for \*(L"Not A Number\*(R". If your \s-1OS\s0 writes \*(L"U\*(R" or \*(L"\s-1UNKN\*(R"\s0 or something similar that's okay. If something else is wrong, it will probably be due to an error you made (assuming that my tutorial is correct of course :\-). In that case: delete the database and try again. .PP The meaning of the above output will become clear below. .SS "Time to create some graphics" .IX Subsection "Time to create some graphics" Try the following command: .PP .Vb 4 \& rrdtool graph speed.png \e \& \-\-start 920804400 \-\-end 920808000 \e \& DEF:myspeed=test.rrd:speed:AVERAGE \e \& LINE2:myspeed#FF0000 .Ve .PP This will create speed.png which starts at 12:00 and ends at 13:00. There is a definition of a variable called myspeed, using the data from \s-1RRA\s0 \&\*(L"speed\*(R" out of database \*(L"test.rrd\*(R". The line drawn is 2 pixels high and represents the variable myspeed. The color is red (specified by its rgb-representation, see below). .PP You'll notice that the start of the graph is not at 12:00 but at 12:05. This is because we have insufficient data to tell the average before that time. This will only happen when you miss some samples, this will not happen a lot, hopefully. .PP If this has worked: congratulations! If not, check what went wrong. .PP The colors are built up from red, green and blue. For each of the components, you specify how much to use in hexadecimal where 00 means not included and \s-1FF\s0 means fully included. The \*(L"color\*(R" white is a mixture of red, green and blue: \s-1FFFFFF\s0 The \*(L"color\*(R" black is all colors off: 000000 .PP .Vb 5 \& red #FF0000 \& green #00FF00 \& blue #0000FF \& magenta #FF00FF (mixed red with blue) \& gray #555555 (one third of all components) .Ve .PP Additionally you can (with a recent RRDtool) add an alpha channel (transparency). The default will be \*(L"\s-1FF\*(R"\s0 which means non-transparent. .PP The \s-1PNG\s0 you just created can be displayed using your favorite image viewer. Web browsers will display the \s-1PNG\s0 via the \s-1URL\s0 \&\*(L"file:///the/path/to/speed.png\*(R" .SS "Graphics with some math" .IX Subsection "Graphics with some math" When looking at the image, you notice that the horizontal axis is labeled 12:10, 12:20, 12:30, 12:40 and 12:50. Sometimes a label doesn't fit (12:00 and 13:00 would be likely candidates) so they are skipped. .PP The vertical axis displays the range we entered. We provided kilometers and when divided by 300 seconds, we get very small numbers. To be exact, the first value was 12 (12357\-12345) and divided by 300 this makes 0.04, which is displayed by RRDtool as \*(L"40 m\*(R" meaning \*(L"40/1000\*(R". The \*(L"m\*(R" (milli) has nothing to do with meters (also m), kilometers or millimeters! RRDtool doesn't know about the physical units of our data, it just works with dimensionless numbers. .PP If we had measured our distances in meters, this would have been (12357000\-12345000)/300 = 12000/300 = 40. .PP As most people have a better feel for numbers in this range, we'll correct that. We could recreate our database and store the correct data, but there is a better way: we do some calculations while creating the png file! .PP .Vb 6 \& rrdtool graph speed2.png \e \& \-\-start 920804400 \-\-end 920808000 \e \& \-\-vertical\-label m/s \e \& DEF:myspeed=test.rrd:speed:AVERAGE \e \& CDEF:realspeed=myspeed,1000,\e* \e \& LINE2:realspeed#FF0000 .Ve .PP Note: I need to escape the multiplication operator * with a backslash. If I don't, the operating system may interpret it and use it for file name expansion. You could also place the line within quotation marks like so: .PP .Vb 1 \& "CDEF:realspeed=myspeed,1000,*" \e .Ve .PP It boils down to: it is RRDtool which should see *, not your shell. And it is your shell interpreting \e, not RRDtool. You may need to adjust examples accordingly if you happen to use an operating system or shell which behaves differently. .PP After viewing this \s-1PNG,\s0 you notice the \*(L"m\*(R" (milli) has disappeared. This is what the correct result would be. Also, a label has been added to the image. Apart from the things mentioned above, the \s-1PNG\s0 should look the same. .PP The calculations are specified in the \s-1CDEF\s0 part above and are in Reverse Polish Notation (\*(L"\s-1RPN\*(R"\s0). What we requested RRDtool to do is: \&\*(L"take the data source myspeed and the number 1000; multiply those\*(R". Don't bother with \s-1RPN\s0 yet, it will be explained later on in more detail. Also, you may want to read my tutorial on CDEFs and Steve Rader's tutorial on \s-1RPN.\s0 But first finish this tutorial. .PP Hang on! If we can multiply values with 1000, it should also be possible to display kilometers per hour from the same data! .PP To change a value that is measured in meters per second: .PP .Vb 3 \& Calculate meters per hour: value * 3600 \& Calculate kilometers per hour: value / 1000 \& Together this makes: value * (3600/1000) or value * 3.6 .Ve .PP In our example database we made a mistake and we need to compensate for this by multiplying with 1000. Applying that correction: .PP .Vb 1 \& value * 3.6 * 1000 == value * 3600 .Ve .PP Now let's create this \s-1PNG,\s0 and add some more magic ... .PP .Vb 10 \& rrdtool graph speed3.png \e \& \-\-start 920804400 \-\-end 920808000 \e \& \-\-vertical\-label km/h \e \& DEF:myspeed=test.rrd:speed:AVERAGE \e \& "CDEF:kmh=myspeed,3600,*" \e \& CDEF:fast=kmh,100,GT,kmh,0,IF \e \& CDEF:good=kmh,100,GT,0,kmh,IF \e \& HRULE:100#0000FF:"Maximum allowed" \e \& AREA:good#00FF00:"Good speed" \e \& AREA:fast#FF0000:"Too fast" .Ve .PP Note: here we use another means to escape the * operator by enclosing the whole string in double quotes. .PP This graph looks much better. Speed is shown in km/h and there is even an extra line with the maximum allowed speed (on the road I travel on). I also changed the colors used to display speed and changed it from a line into an area. .PP The calculations are more complex now. For speed measurements within the speed limit they are: .PP .Vb 2 \& Check if kmh is greater than 100 ( kmh,100 ) GT \& If so, return 0, else kmh ((( kmh,100 ) GT ), 0, kmh) IF .Ve .PP For values above the speed limit: .PP .Vb 2 \& Check if kmh is greater than 100 ( kmh,100 ) GT \& If so, return kmh, else return 0 ((( kmh,100) GT ), kmh, 0) IF .Ve .SS "Graphics Magic" .IX Subsection "Graphics Magic" I like to believe there are virtually no limits to how RRDtool graph can manipulate data. I will not explain how it works, but look at the following \s-1PNG:\s0 .PP .Vb 10 \& rrdtool graph speed4.png \e \& \-\-start 920804400 \-\-end 920808000 \e \& \-\-vertical\-label km/h \e \& DEF:myspeed=test.rrd:speed:AVERAGE \e \& CDEF:nonans=myspeed,UN,0,myspeed,IF \e \& CDEF:kmh=nonans,3600,* \e \& CDEF:fast=kmh,100,GT,100,0,IF \e \& CDEF:over=kmh,100,GT,kmh,100,\-,0,IF \e \& CDEF:good=kmh,100,GT,0,kmh,IF \e \& HRULE:100#0000FF:"Maximum allowed" \e \& AREA:good#00FF00:"Good speed" \e \& AREA:fast#550000:"Too fast" \e \& STACK:over#FF0000:"Over speed" .Ve .PP Remember the note in the beginning? I had to remove unknown data from this example. The 'nonans' \s-1CDEF\s0 is new, and the 6th line (which used to be the 5th line) used to read 'CDEF:kmh=myspeed,3600,*' .PP Let's create a quick and dirty \s-1HTML\s0 page to view the three PNGs: .PP .Vb 7 \& Speed \& Speed in meters per second \&
\& Speed in kilometers per hour \&
\& Traveled too fast? \& .Ve .PP Name the file \*(L"speed.html\*(R" or similar, and look at it in your web browser. .PP Now, all you have to do is measure the values regularly and update the database. When you want to view the data, recreate the PNGs and make sure to refresh them in your browser. (Note: just clicking reload may not be enough, especially when proxies are involved. Try shift-reload or ctrl\-F5). .SS "Updates in Reality" .IX Subsection "Updates in Reality" We've already used the \f(CW\*(C`update\*(C'\fR command: it took one or more parameters in the form of \*(L"