Creating Web Plots on Demand

Mr. Pruett tells us how his company creates on-the-fly plots of database information for web display.
Cleaning Up the GIF Files

One minor issue remains. We create a new GIF file each time our Perl program is run, so we must get rid of them when they're no longer needed.

In my case, each night I simply run a script under crontab that deletes any GIF files created during the day. Since most of our applications are run during the daytime hours, the chances of deleting a GIF file I still need are very small.

This scheme might not be acceptable in every situation, so you may need to devise a different way to clean up the GIF files that collect on your web server. [The find command will fit this purpose admirably—Ed]

Reviewing the Steps

While I've provided a specific example of on-demand plotting, the techniques used can be applied to any type of data you might want to plot. If you can extract the data to a simple text file, and if the data lends itself to two- or three-dimensional plotting, you can deliver it to the Web. The basic steps are always the same:

  • Build a text file with the data to plot.

  • Build a gnuplot command file.

  • Run gnuplot to build the plot in PPM format.

  • Convert the plot to GIF format using ppmtogif.

  • Build an HTML page with the image tag and send it to the browser.

Tying together tools like gnuplot and NetPBM to quickly build a useful program shows that software doesn't necessarily have to come packaged in the latest object-oriented component, tied together with ActiveX or CORBA. Often, good solid tools, text files and a touch of Perl will more than suffice to do the job.

Resources

Mark Pruett received his M.S. in computer science from Virginia Commonwealth University. Mark is a programmer who writes about programming. He hopes some day to be a writer who writes about how to write program documentation. He can be reached at pruettm@vancpower.com.

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