I'd second this. We've been using GRaphael for the reporting graphs on MinuteDock, and we had to manually hack in a bunch of things - like rotating the labels on the x-axis correctly if they were too long, and even drawing both axes on some graph types.

It's worth the effort though - works on iPhone/iPad out of the box, and is super snappy even on OS X (unlike most Flash charting solutions, including Google Viz.)