Dietrich - 3 months ago 21

Python Question

In matplotlib, I would like draw an filled arc which looks like this:

The following code results in an unfilled line arc:

`import matplotlib.patches as mpatches`

import matplotlib.pyplot as plt

fg, ax = plt.subplots(1, 1)

pac = mpatches.Arc([0, -2.5], 5, 5, angle=0, theta1=45, theta2=135)

ax.add_patch(pac)

ax.axis([-2, 2, -2, 2])

ax.set_aspect("equal")

fg.canvas.draw()

The documentation says that filled arcs are not possible.

What would be the best way to draw one?

Answer

@jeanrjc's solution almost gets you there, but it adds a completely unnecessary white triangle, which will hide other objects as well (see figure below, version 1).

This is a simpler approach, which only adds a polygon of the arc:

Basically we create a series of points (`points`

) along the edge of the circle (from `theta1`

to `theta2`

). This is already enough, as we can set the `close`

flag in the `Polygon`

constructor which will add the line from the last to the first point (creating a closed arc).

```
import matplotlib.patches as mpatches
import matplotlib.pyplot as plt
import numpy as np
def arc_patch(center, radius, theta1, theta2, ax=None, resolution=50, **kwargs):
# make sure ax is not empty
if ax is None:
ax = plt.gca()
# generate the points
theta = np.linspace(np.radians(theta1), np.radians(theta2), resolution)
points = np.vstack((radius*np.cos(theta) + center[0],
radius*np.sin(theta) + center[1]))
# build the polygon and add it to the axes
poly = mpatches.Polygon(points.T, closed=True, **kwargs)
ax.add_patch(poly)
return poly
```

And then we apply it:

```
fig, ax = plt.subplots(1,2)
# @jeanrjc solution, which might hide other objects in your plot
ax[0].plot([-1,1],[1,-1], 'r', zorder = -10)
filled_arc((0.,0.3), 1, 90, 180, ax[0], 'blue')
ax[0].set_title('version 1')
# simpler approach, which really is just the arc
ax[1].plot([-1,1],[1,-1], 'r', zorder = -10)
arc_patch((0.,0.3), 1, 90, 180, ax=ax[1], fill=True, color='blue')
ax[1].set_title('version 2')
# axis settings
for a in ax:
a.set_aspect('equal')
a.set_xlim(-1.5, 1.5)
a.set_ylim(-1.5, 1.5)
plt.show()
```

Result (version 2):