This is an entirely javascript-based implementation of gnuplot v.4.6.3 based on the work here: gnuplot-JS. I updated it some to use CodeMirror for the editor and setup the output image to allow resizing.
The output file must be 'out.svg' (and use svg terminal). You can "upload" (not uploaded, just stored in browser memory) data files with the button below.

Optional: Grant the program read-access to a specific file on your local computer. This file is _not_ uploaded anywhere. Use it for data files that you want to use in your script.