Unfortunately, fields have to be completely identical in type,name and label to be included in Cross Project analysis graphs.
The User's Guide does convey this and I have verified this in the lab. (See Reference Below)
Quality Center v10 - Users Guide - Chapter 41 - Page 581
Considerations for Creating Cross-Project Graphs
''Every time you add or remove projects in your graph, Quality Center
updates the Filter Condition list, and the drop-down lists of the following
boxes: X-Axis, Y-Axis, and Grouped By. The lists contain only fields that are
identical in all the selected projects. Identical fields are fields that have the
same label and the same data type.''