In the May 2009 release, SFDC implemented changes to the reporting engine. Most of these changes were for the better, but they did break the ability to do some basic graph concepts - in this case, a graph showing revenue by forecast category over time.
These graphs are a staple of business reporting. Time on the X axis, Revenue on the Y axis. Until the May 2009 release, it was possible to do this correctly, when viewing the forecast categories, with the most 'solid' revenue type (closed) at the bottom of the stack, then stacked upwards in descending order.
After the May 2009 release, this graph is no longer possible. A stacked column graph is now forced to show closed revenue at the top of the stack, with the least certain revenue streams at the bottom of the graph.
This should at worst be a configurable option, allowing users to select the order components are displayed in.