Ok... Here's how it happened:
1) I opened a file that has both expired and not-expired cards:
(http://arqui3d.com/odd1.gif)
Notice that it's set to "Repeat advisor".
2) Opened the RM... Notice how the bar graph looks.
(http://arqui3d.com/odd2.gif)
3) And clicked on "Examine expired cards":
(http://arqui3d.com/odd3.gif)
Notice how the graph changes. And the file is still set to "Repeat Advisor"...
Notice how all stacks are selected when pressing the button.
I vote for "no bug".
Well, yes, it was just another... PEBKAC! (Problem Exists Between Keyboard And Chair)...
I really though that the graph was always based on the whole file... Had no idea it was related to the selection.
By the way, I don't know what it is, but with Stackz! 7 I've finally come to appreciate the Learn dialog. Before that, I always used the Test dialog to learn ;D .