visualize rails api

I am looking for ways to better understand the rails api.

I found a tool used by Drupal that visualizes the drupal api in a graph
that demonstrates the relationship between methods calling one another.

See a screencast on it: http://blip.tv/file/431841

The value I saw in it was that it increased the clarity of how methods
relate to one another. When I debug rails (have been using Netbeans to
do this), it's hard to track when new files are opened and the overall
flow is difficult to track as method calls move across the framework.

Is there any similar tool for Ruby or Rails? Or any other type of
approach that might help me better navigate the API?

Thanks,

John