See this thread:
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I don't know about problem 1, but I has the same problem #2 (White Gap between 1st and 2nd Series). And found a workaround
I stared at drawItem on StackedXYAreaRenderer for a while and it looked like the base of each series was drawn from the series proceeding and hence the order to go through the series would be forward from 0 to n.
But the XYPlot drawItem iteration order default is REVERSE from n to 0.
So as a workaround I had luck by calling the following after creating the plot with the StackedXYAreaRenderer:
And that seemed to solve the white gap problem.
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I have also just encountered this problem and we really won't be able to use this type of chart until a fix is released. :-(
I have just found this problem and markdown40's workaround worked.
I've also discovered that not only a gap is formed between 2 series, but when a 3rd or a 4th series is added, the second series becomes the gap, and the colors of the following series are shifted: the 3rd is filled with what the 2nd's color would be, the 4th with the 3rd's and so on. You may see a code snippet that replicates this issue on http://gist.github.com/18641