John C. Gunther :
Client-side GChart 2.5 adds a new canvas-rendering option for faster, better-looking, line, pie, and area charts with RGBA (alpha-transparent) colors.
This release also adds centered/inside ticks, improved chart print-ability, and more.
GChart performs its rendering via GWT Widgets, but GWT does not come with a canvas Widget standard. So, you'll have to implement an interface that teaches GChart how to create thecanvas Widgets it needs to render your charts.
Sounds complicated, but, if you use the GWT incubator project's GWTCanvas, it's easy. Just add gwt-incubator.jar to your project build path and .gwt.xml file as usual. Then copy about 6 lines of standard boilerplate into your application's EntryPoint class. Done.
The live demo provides a working example (follow the link at the bottom of the page to see how the live demo includes the required boilerplate).
Note that, if you are OK with its default HTML-only rendering, GChart still works just fine without such canvas-related interfacing. You won't escape the much-maligned banded-filled pie slices, but that's life.
Canvas rendered, and still 100% pure GWT Java?
Granted, GWTCanvas contains a heck of a lot of Javascript that isn't part of the standard GWT distro. So, the claim that GChart is strictly layered on plain-vanilla GWT, though technically true (there's no Javascript in GChart itself), is a bit misleading if you need to use the canvas rendering option.
On the other hand, GChart uses GWTCanvas in the same 'who cares what browser' manner that it uses, say, the standard GWT Grid Widget. Moreover, there is a reasonably good chance that some future GWT distro will include a better, improved, GWTCanvas.
So, GChart is still designed around the idea of being strictly layered on top of Widgets and such that capture the browser-specific differences at the lowest possible level. This approach assures that your chart library isn't just open source, it's 'you can easily read, change, and debug the source'.
One easily quantified advantage of GChart's approach: when I recompiled the live demo in transitioning from 1.5.3 to 1.6.4, I noticed that the charts were rendering about 15% faster.
Homepage(live demo, downloads, docs): http://gchart.googlecode.com
Release Notes: http://gchart.googlecode.com/svn/trunk/doc/com/googlecode/gchart/client/doc-files/gchart2p5features.html
