Essentially all you need is a version of the Gnuplot data plotting program which can produce PNG output. The standard Gnuplot distribution does not include the code necessary to do this.
The Portable Network Graphics format (PNG) is a graphic file format which is intended to replace the GIF file format. PNG is supported by most modern browsers, including Internet Explorer 4.0 and later.
The most recent production release of Gnuplot as of this writing is 4.0.0. You can get it from the master archive at Gnuplot.info. Before building it, you need to build and install the Zlib compression library which is required by libpng, and then also build and install libpng. Once you have installed the libraries and their header files in /usr/lib and /usr/include respectively, a simple "./configure && make" in Gnuplot's source directory should detect that the libraries are installed and compile the program.
Or, an easy way is to accomplish this is to get a single tarball containing Gnuplot 4.0.0, Zlib, and libpng. After extracting the .tar.gz file, run the make_all.sh script, which will attempt to do everything you need to do to build a gnuplot binary linked with PNG support.
An even easier way is to download a precompiled version of Gnuplot for your platform.