Creating Web Plots on Demand
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]
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.
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