I've been trying to get pretty charts to work in GWT on our internal network.
Playing around with GWT-Ext's charts is nice, but it requires flash and is really messy to control (it seems buggy, in general).
I'd like to hear about something that works with the least amount of dependencies and it also must work without a connection to the web (so, Google' charts API isn't a solution).
Edit: Indeed, I would rather a library that is all client-side.
Do you want something that has a server side component or entirely client driven? The best ones I have seen are all flash, alas. I have done little tricks with JS and GWT before, but there is only sophisticated I will get before I go hunting for a library to do it for me.
There is one open source api for charts in GWT hosted on http://code.google.com/p/gwt-rcharts/ . The API works on SVG/VML specification. You may find it quite easy to implement and use. You may find the demo at http://gwt-rcharts.appspot.com/
Google's charts actually come in two flavours, and one of them does not require interaction with Google's servers - so should satisfy your needs.
Google Image Charts is the API you are thinking of, which is an API on Google's servers that returns images.
Google Interactive Charts is a client side javascript API that renders entirely within the browser: Google Interactive Charts
Google provides a GWT wrapper for the interactive charts: GWT Visualization API
It's not all rainbows and unicorns and you can find chart libs out there that make nicer charts, but it's pretty solid, works on all major browsers and we've been using it successfully for quite a while.
There is also "sparklines" - they are available in lots of flavours (very simple charts though).
gchart looks seriously awesome. Go with it !
Googling for "GWT +sparklines" has gotten me to gchart, which seems like what I need.
From what I understand - it's all client side and requires nothing more than their JAR file.