I have a problem plotting text on a graph that's been rotated using an Affine transform. I think I can demonstrate the cause of the problem with the following code snippet.
from matplotlib.transforms import Affine, Value
a = Affine(Value(1), Value(1), Value(-1), Value(1), Value(0), Value(0))
xy = (123, 456)
Result is not 123, 456 as I expect but -456, 456.
I think the problem is in _transform.cpp. This patch (from HEAD revision in SVN) gives the correct result.
< _ibval = scale*_cval;
< _icval = -scale*_bval;
> _ibval = -scale*_bval;
> _icval = -scale*_cval;