An interactive graphing tool, using g.raphael, for exploring relationships among datasets. The tool allows you to add multiple datasets onto a graph with a single (automatic) y-axis scale, and also allows you to layer an arbitrary number of these graphs, with different y-axis scales, on top of each other. A variable can also be chosen for the x-axis, to draw scatterplots.
You can try out a demo at http://ejucovy.github.com/graphtool/sample.html.
(Some) instructions are available.