setCircular(true) does not always work on a pie chart because sometimes the transform set on the graphics context passed in has different X and Y scales which cause an ellipse to be drawn.
A work around is to call chart.setMaximumDrawWidth() and chart.setMaximumDrawHeight() with values bigger than the panel size like Integer.MAX_VALUE but I'm not sure what the side effects of this are.
This scaling effect (by the ChartPanel) will distort shapes in the charts (and fonts too) but you can turn it off as you discovered. This isn't strictly a bug.
The only side effect of increasing the maximumDrawHeight and Width in the ChartPanel is that the offscreen buffer will have to be larger (and thus consume more memory)...but it will avoid the distortion that you see.
There should be a way of setting the scaling to be uniform (same x and y scales), and / or the documentation should be changed to point out this side effect of scaling and how to overcome it.
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