A dataset of two series with values like [[0, 1], [1, 2], [1, 3], [2, 4]] and [[0, -1], [1, -2], [1, -3], [2, -4]] will cause a weird rendering of the difference between the two series. This is caused by the the two tuple pairs ([1, 2], [1, 3]) and ([1, -2], [1, -3]).
This could be a situation where someone could withdraw a sum of money from a saving account on one day. This would show as a step-effect. Simply removing the lowest value would give a different image and would not display the jump.
To work around the error I had to check for similar x-values in the input and add a small delta. The code attached in the zip file will show how to reproduce and work around this.
Three pics included.
Introduce delta works around bug: http://img171.imageshack.us/img171/258/xydifferencerendererint.png
Remove value: http://img710.imageshack.us/img710/8521/xydifferencerendererrem.png
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