Gplot provides a convenient front-end for gnuplot. It accepts a large set of options to generate a single plot of one or more sets of data (overlaid as necessary).
Web based PHP interface to communicate to Maxima computer algebra system in the server. It takes user input, send it to Maxima, and show the output back to website. It could send almost any commands including 'plot2d' and 'plot3d' to plot graphs.