I think using Google Chart API is much better than using these plugins. Lots of chart types, easy to make, gives high-quality result, and support all browsers that can display images.
If you ask me (I maintain a fork of flot on github) there is no reason to use a client-side library for graphing unless the graphs are dynamic. Requesting/rendering/etc tons of images is painful, flot can add and remove data on the fly and re-render -- and that's not even getting into the potential for hovering effects and the like (flash-style).
Flot also supports all reasonable browsers (including some unreasonable ones, like IE6). If you find a bug in either the 'official' flot or my fork (or any of the forks of my fork :)) then you should be able to report it and have it fixed quickly enough (or, fix it yourself, the code isn't especially complicated).
A couple months ago I built a reporting tool for a client using Flot, only to have to switch over to Google Charts after users started complaining.
Flot, and all other Canvas-based JS charting solutions, are really impressive applications of Canvas, but until browsers start supporting Canvas copy/paste better I'm hesitant to use them for graphing apps. Image-rendering graphing solutions support most browsers and can be easily copy/pasted into other applications, with little additional overhead.
