We have looked at this issue further. Given the wide variety of label sizes, color and layout conditions exist in legend chart design make it problematic to designing a legend that fits every use case. The workable solution is to design a legend widget that could fit any size of label, color and positioning for the developer. Our goal was to provide basic legend functionality in the charts for the common use cases and since a custom legend widget could be built to fit any legend condition by the developer, we are going to close this issue.
That said, using other GXT widgets, you can create functionality like what you are describing, and have it be as flexible as you want. For example, Colin provides some examples of how to go about building a standalone widget in this thread and may be useful for more information. http://www.sencha.com/forum/showthread.php?250682
The workaround would be to use something like ListView which could be used to provide the legend infrastructure and the ListView selection model which could be used for event observation. I would also check out the the dashboard http://www.sencha.com/examples/#ExamplePlace:dashboard because it provides a good example of how to link up charts to other widgets and help give some direction to external widget linking. Another thought is using the draw library could provide another nifty way to create a widget as well.
At the moment I'm going to close the issue. If you have some more questions on customized legend widget hit the reply.