How to rotate matplotlib.patches.Polygon?

Question

How to rotate matplotlib.patches.Polygon? The output from the code below shows nothing.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.patches as patches
import matplotlib as mpl
fig, ax = plt.subplots(figsize=(10,3))
x = [0.3,0.6,.5,.4]
y = [0.7,0.7,0.9,0.9]

trapezoid = patches.Polygon(xy=list(zip(x,y)), fill=False)

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

trapezoid.set_transform(t_end)

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

plt.show()

Did you check out posts such as [Unable to rotate a matplotlib patch object about a specific point using rotate_around( )](https://stackoverflow.com/questions/15557608/unable-to-rotate-a-matplotlib-patch-object-about-a-specific-point-using-rotate-a) and [Matplotlib: rotating a patch](https://stackoverflow.com/questions/4285103/matplotlib-rotating-a-patch) — JohanC, Jan 14 '21 at 08:33
I tried it. I edited the code in my question. What is wrong that the output does not show the rotated object? — Elena Greg, Jan 14 '21 at 09:12

score 1 · Accepted Answer · answered Jan 14 '21 at 12:12

When composing the tranformations, you must use t_end = t + t_start instead of t_start + t. The + operator is overloaded: a + b means to first apply a and then apply b. For affine transformations this means the matrix product B@A where A and B are the transformation matrices of a and b respectively. The matrix product is not commutative.

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

fig, ax = plt.subplots(figsize=(10,3))
x = [0.3,0.6,.5,.4]
y = [0.7,0.7,0.9,0.9]

trapezoid = patches.Polygon(xy=list(zip(x,y)), fill=False)
ax.add_patch(copy.copy(trapezoid))

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

trapezoid.set_transform(t_end)
ax.add_patch(trapezoid)

plt.show()

How to rotate matplotlib.patches.Polygon?

1 Answers1