CODEWITHSUNDEEP
pythonmatplotlib
Killer
Killer

Reputation: 417

Strange Behavior of Python's Matplotlib Module - Plotting a Circle

I am currently writing a program which must generate a set of plots. EACH plot must have 3 concentric circles on it whose radii are determined by a data set. Further, another red colored circle must also be added which can have a different centre. However, I ran into various problems. Unless the radius of the circle/s is/are too large, I should see 3 black and 1 red circle on the plot but I don't.

I isolated the piece of code that makes the plot and here it is - 

import matplotlib.pyplot as plt

fig1 = plt.figure(1, figsize=(6,6))
plt.xlim(-30,30)
plt.ylim(-30,30)

rcircle1 = plt.Circle( (0,0), 6.0, edgecolor="black", facecolor="white")
rcircle2 = plt.Circle( (0,0), 12.0, edgecolor="black", facecolor="white")
rcircle3 = plt.Circle( (0,0), 18.0, edgecolor="black", facecolor="white")
bcircle = plt.Circle( (8.5,-5.8)  ,2,  edgecolor="red", facecolor="white")

ax = fig1.gca()
ax.add_artist(rcircle1)
ax.add_artist(rcircle2)
ax.add_artist(rcircle3)
ax.add_artist(bcircle)

fig1.savefig("Model.png", dpi=150)

The output for above is -

Output of above code.

I tried looking into various Class variables associated with Circle() and add_artist() but unable to find something that might be affecting this behavior.

My current work around is the following code - 

import numpy as np
import matplotlib.pyplot as plt

th = np.arange(-3.14,3.14,0.01)

fig1 = plt.figure(1,figsize=(6,6))
plt.xlim(-30,30)
plt.ylim(-30,30)

plt.plot( 6*np.cos(th), 6*np.sin(th), color="black")
plt.plot( 12*np.cos(th), 12*np.sin(th), color="black")
plt.plot( 18*np.cos(th), 18*np.sin(th), color="black")
# (8,5, -5,8)
plt.plot( 2*np.cos(th) + 8.5, 2*np.sin(th) - 5.8, color="red")

fig1.savefig("Hard.png", dpi=150)

The output generated by the above code is exactly what I want to be!

enter image description here

While this does work, it defeats the purpose of having Circle() like methods in matplotlib. Can anyone comment why the first code is not working as I expect it to be?

Upvotes: 3

Views: 1009

Answers (1)

Joe Kington
Joe Kington

Reputation: 284890

Your problem is the facecolor argument. You're adding the biggest circle last, and it has an opaque center.

In the second example you're plotting a line, not a "filled" circle.

Either change the order you add the circles in (or supply a zorder kwarg), or pass in facecolor='none' (Note: it's the string "none", not the object None) to get an "unfilled" circle.

Upvotes: 6

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