I used JfreeChart 0.9.21 till some days ago. I made some 'special' graphs with XYDifferenceRenderer (they are river profiles with water level).
When I moved to the latest version of the library my old code breaked, because the data I plot are not compatible with the new algorithm. That's bad !
Don't know how many people did a so 'special' use of the library, perhaps it is worth to insert a flag and let the user choose if he wants to use the old algorithm to compute the difference between the series.
Here are the XY data:
First serie: (0,90) (10,60) (20,60) (20, 80) (25, 80) (25, 60) (35, 60) (45, 90)
Second serie: (0, 70) (10,70) (20,70) (20, 70) (25, 70) (25, 70) (35, 70) (45, 70)
As you can see the first serie has two steps and they breaks the algorithm.