I wrote this mainly for my own convenience -- there are times when you don't want to wait for Matlab to open, or even use Python's matplotlib.
Your data is plotted in real time as you type, or you can copy/paste in your data in csv format.
What features do you want - uploading text files? scatter plots? zooming? comparing multiple trends? plotting an arbitrary function? anything else? I'm all ears.
Just one of those projects. Stanford's protovis library does the real work. And here's a direct link to the page.