Setting annotation background covers arrow

[ndevenish]

Setting a background on the annotation covers the arrow also. This didn't happen in 1.4.2. I've been using it to separate the text from any plot data points in cases where they overlap. Resolvable for now by redrawing an empty annotation with an arrow.

Pre 1.4.2:

Current:

Example code:

import matplotlib.pyplot as plt
import numpy
a = numpy.zeros(100)
a[::2] = 1
plt.figure(figsize=(5.9,4.4))
plt.plot(a)
shared_args = {"xytext":(-15,0), "va":'center', "ha":"right", "textcoords":'offset points', "arrowprops":{"arrowstyle": "->"}}
plt.annotate("Test", xy=(20,0.5), **shared_args)
plt.annotate("Test", xy=(20,0.4), backgroundcolor="w", **shared_args)
plt.show()

And the actual introducing commit, in the middle of what appears to be other restructuring 9d1c876

[tacaswell]

Suspect that this is related to #4139 #4140 and #4178.

@efiring took a look at these issues and I have been avoiding looking into these issues as this code is more than a bit hairy.

[ndevenish]

Hmm, yes it is very closely related to #4139 - the root cause is the same, but with an extra ordering issue in this one.

[tacaswell]

This appears to be fixed, have not done forensics to sort out by what.