Text rotation seems to occur about different points depending on the sign of the angle passed:

import matplotlib.pyplot as plt
from matplotlib import __version__
print(__version__)
#2.0.0b1+1800.g5c1e64d.dirty

fig, ax = plt.subplots()
xx = 0.5, 0.5
ax.plot(*xx, 'rx')
blk = '\u2588' * 10  #unicode blocks
ax.text(*xx, blk, rotation=-45, color='b')  # rotation about point xx
ax.text(*xx, blk, rotation=45, color='r')   # rotation about rhs of textbox

So essentially: A negative angle rotates about the lower left corner of text box, while a positive angle rotates about the lower right corner of text box. Is this desired behaviour? Personally I find it somewhat confusing and would expect rotations to have the same origin, regardless of the sign of the angle passed.