This question is about toolkits which draw graphs (those things composed of vertices and edges), not generic plots.
The tool must be able to get a set of vertices and edges, calculate their layout, and display them either using the canvas tag, or SVG, that is be compatible with HTML5.
Flash-based tools and java applets are out.
to serve as unified benchmark, please measure and report how long it takes for the toolkit to layout and draw a Barabasi-Albert graph with 100 vertices and 5 edges per node.
This python lybrary shows how to generate it and exports it to many formats:
http://networkx.lanl.gov/tutorial/tutorial.html#graph-generators-and-graph-operations
Please indicate browser and CPU.
JavaScript InfoVis Toolkit is awesome and i have not seen a competitor to it.