Unable to rotate a matplotlib patch object about a specific point using rotate_around( )

Question

I am trying to rotate a matplotlib rectangular patch object about a specific point using the rotate_around() and rotate_deg_around() functions. However, the patch is always rotating about the origin. I am not sure how to ensure that the patch object rotates about a specific point.

Here code is as follows:

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)
ax.set_xlim(-0.05,1);ax.set_ylim(-0.05,1);
grid('on');

#Rotate rectangle patch object
ts = ax.transData
tr = mpl.transforms.Affine2D().rotate_deg_around(0.2,0.5,10)
t= ts + tr

rec0 = patches.Rectangle((0.2,0.5),0.25,0.2,alpha=0.5)
ax.add_patch(rec0)

#Rotated rectangle patch
rect1 = patches.Rectangle((0.2,0.5),0.25,0.2,color='blue',alpha=0.5,transform=t)
ax.add_patch(rect1);

#The (desired) point of rotation
ax.scatter([0.0,0.2],[0.0,0.5],c=['g','r'],zorder=10)
txt = ax.annotate('Desired point of rotation',xy=(0.2,0.5),fontsize=16,\
xytext=(0.25,0.35),arrowprops=dict(arrowstyle="->",connectionstyle="arc3,rad=-.2"))
txt2 = ax.annotate('Actual point of rotation',xy=(0.0,0.0),fontsize=16,\
xytext=(0.15,0.15),arrowprops=dict(arrowstyle="->",connectionstyle="arc3,rad=.2"))

plt.show()

Here is the output of the above code:

I have also tried to do translate, rotate_about_origin, and translate_back. However, the translate transform was not working either. Any help/example of simple translation would also be very useful.

Thank you.

Answer 1

The coordinates you rotate around are not the data coordinates. You have to transform them first, i.e.

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)
ax.set_xlim(-0.05,1);ax.set_ylim(-0.05,1);
plt.grid('on');

#Rotate rectangle patch object
ts = ax.transData
coords = ts.transform([0.2, 0.5])
tr = mpl.transforms.Affine2D().rotate_deg_around(coords[0], coords[1], 10)
t= ts + tr

rec0 = patches.Rectangle((0.2,0.5),0.25,0.2,alpha=0.5)
ax.add_patch(rec0)

#Rotated rectangle patch
rect1 = patches.Rectangle((0.2,0.5),0.25,0.2,color='blue',alpha=0.5,transform=t)
ax.add_patch(rect1);

#The (desired) point of rotation
ax.scatter([0.0,0.2],[0.0,0.5],c=['g','r'],zorder=10)
txt = ax.annotate('Desired point of rotation',xy=(0.2,0.5),fontsize=16,\
xytext=(0.25,0.35),arrowprops=dict(arrowstyle="->",connectionstyle="arc3,rad=-.2"))
txt2 = ax.annotate('Actual point of rotation',xy=(0.0,0.0),fontsize=16,\
xytext=(0.15,0.15),arrowprops=dict(arrowstyle="->",connectionstyle="arc3,rad=.2"))

plt.show()

Edit:

Apparently, the code only works for the interactive display, but not when the window is resized or the figure is saved. Compare these two images: