Matplotlib: rotating a patch

Question

I wanted to rotate a Rectangle in matplotlib but when I apply the transformation, the rectangle doesn't show anymore:

rect = mpl.patches.Rectangle((0.0120,0),0.1,1000)
t = mpl.transforms.Affine2D().rotate_deg(45)
rect.set_transform(t)

is this a known bug or do I make a mistake?

Answer 1

The patch in the provided code makes it hard to tell what's going on, so I've made a clear demonstration that I worked out from a matplotlib example:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as patches
import matplotlib as mpl

fig = plt.figure()
ax = fig.add_subplot(111)

r1 = patches.Rectangle((0,0), 20, 40, color="blue", alpha=0.50)
r2 = patches.Rectangle((0,0), 20, 40, color="red",  alpha=0.50)

t2 = mpl.transforms.Affine2D().rotate_deg(-45) + ax.transData
r2.set_transform(t2)

ax.add_patch(r1)
ax.add_patch(r2)

plt.xlim(-20, 60)
plt.ylim(-20, 60)

plt.grid(True)

plt.show()

Answer 2

Apparently the transforms on patches are composites of several transforms for dealing with scaling and the bounding box. Adding the transform to the existing plot transform seems to give something more like what you'd expect. Though it looks like there's still an offset to work out.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as patches
import matplotlib as mpl
fig = plt.figure()
ax = fig.add_subplot(111)

rect = patches.Rectangle((0.0120,0),0.1,1000)

t_start = ax.transData
t = mpl.transforms.Affine2D().rotate_deg(-45)
t_end = t_start + t

rect.set_transform(t_end)

print repr(t_start)
print repr(t_end)
ax.add_patch(rect)

plt.show()