Creating Web Plots on Demand
gnuplot will automatically label the x and y axes with values from our data set. If the maximum number of web hits for any time interval in the data set was 48, then gnuplot might create tic marks on the y axis at 0, 10, 20, 30, 40 and 50. This would be fine for the y axis, but remember that the x axis contains a calculated time value that might not make much sense to a person reading our graph.
Luckily, we can override the default tic marks and create our own. That's the job of the for loop at the beginning of section five. We create a text string that will be embedded in the gnuplot command file we are about to create.
gnuplot creates plots based on a set of commands you provide it. Actually, gnuplot has only two commands for plotting data, plot and splot, and we'll be using the simpler of the two in our program. The other command we'll use is set to enable and disable particular options and features in gnuplot. The Perl print statement in section five of Listing 1 handles writing a gnuplot command file to disk.
Those not overly familiar with Perl may find this variation of the print statement somewhat confusing. Let's look at that line:
print GPFILE <<EOM;
This print command tells Perl to write every line in the Perl script following it to the file opened with the GPFILE file handle. It stops printing lines to the file when Perl encounters the line in the script consisting only of the letters EOM. So all the lines in section five following the print statement and continuing to the EOM line are not Perl statements at all, but are gnuplot commands that will be written by Perl to the gnuplot command file.
The print command will also substitute the Perl variable references with their values, so the line:
set title "Web Server Accesses $mon $day, $year"
might be written to the file as:
set title "Web Server Accesses Aug 12, 1997"
Now that we have a data file and a gnuplot command file, how do we get our plot? And how do we show it to the user? This turns out to be the easy part.
First, we need to talk a bit more about gnuplot. I mentioned earlier that gnuplot can display to a variety of devices, including X terminals. But gnuplot can also write a file in portable pixmap format (PPM). In section five of the Perl script, note that we write the gnuplot command:
set term pbm color
to the gnuplot command file. This tells gnuplot to write the output to a file.
However, this only takes us part of the way. Web browsers don't know what to do with PPM files; they usually want either a GIF or JPEG file. That's where the NetPBM package comes in. This is a set of command line utilities that convert from one format to another. One of those tools is exactly what we're looking for: the aptly named ppmtogif. ppmtogif is a simple filter program. It takes a PPM image file from standard input and writes a GIF file to standard output. Since most web browsers support GIF, ppmtogif fits our needs perfectly.
Thus, section six turns out to be almost trivial. We use a Perl system call to run our UNIX commands, then run gnuplot using the command file we created in section five (which in turn will read the data file we built in section four). gnuplot sends the graphic in PPM format to standard output, and the ppmtogif filter turns it into a GIF file that is redirected to a disk file. We now have our graph in GIF format on disk.
The final step (section 7 of Listing 1) is to display the graph in the user's web browser. Remember this is a CGI program, so the browser that invoked the CGI Perl script is waiting to receive something from our web server. What we'll deliver is a brief HTML page containing an HTML image tag that points to the GIF file we just created.
We use the same technique, a multi-line print statement, to create the HTML the browser will receive. Note that we must prepend a “content-type” preamble to the data we send back to the browser. This lets it know to expect an HTML page, rather than some other type of data.
The user generates the plot by typing in the URL of the CGI script in their browser. If the server is called myserver and our script is saved as usage_graph_cgi in the web server's cgi-bin directory, then they would type:
To specify a date other than today, the user appends the date to the end of the URL, separated by a question mark:
http://myserver/cgi-bin/usage_graph_cgi?11/Sep/1997If all goes well, and there's no reason why it shouldn't, the user should see a page that looks something like Figure 1.
|Red Hat Enterprise Linux 7.1 beta available on IBM Power Platform||Jan 23, 2015|
|Designing with Linux||Jan 22, 2015|
|Wondershaper—QOS in a Pinch||Jan 21, 2015|
|Ideal Backups with zbackup||Jan 19, 2015|
|Non-Linux FOSS: Animation Made Easy||Jan 14, 2015|
|Internet of Things Blows Away CES, and it May Be Hunting for YOU Next||Jan 12, 2015|
- Designing with Linux
- Wondershaper—QOS in a Pinch
- Red Hat Enterprise Linux 7.1 beta available on IBM Power Platform
- Internet of Things Blows Away CES, and it May Be Hunting for YOU Next
- Ideal Backups with zbackup
- Slow System? iotop Is Your Friend
- New Products
- 2014 Book Roundup
- Hats Off to Mozilla
- January 2015 Issue of Linux Journal: Security
Editorial Advisory Panel
Thank you to our 2014 Editorial Advisors!
- Jeff Parent
- Brad Baillio
- Nick Baronian
- Steve Case
- Chadalavada Kalyana
- Caleb Cullen
- Keir Davis
- Michael Eager
- Nick Faltys
- Dennis Frey
- Philip Jacob
- Jay Kruizenga
- Steve Marquez
- Dave McAllister
- Craig Oda
- Mike Roberts
- Chris Stark
- Patrick Swartz
- David Lynch
- Alicia Gibb
- Thomas Quinlan
- Carson McDonald
- Kristen Shoemaker
- Charnell Luchich
- James Walker
- Victor Gregorio
- Hari Boukis
- Brian Conner
- David Lane