Fix variable annotation in symlog mode

[nikohansen]

Like invoked by

cma.plot(xsemilog=True)

[nikohansen]

gca().get_transform().inverted() may be useful to achieve this, see also the class documentation of the value of gca().get_transform() here.

subplot(2, 3, 1).axes._yaxis_transform.inverted().transform(...)

asks a 2-D array input, not sure what that does.

gca().transScale

may be relevant.

[nikohansen]

As a small improvement, the annotation in the next version will be at the y-value instead of equally spaced on the linear scale.