Plot Ellipse with matplotlib.pyplot (Python)

Question

Sorry if this is a stupid question, but is there an easy way to plot an ellipse with matplotlib.pyplot in Python? I was hoping there would be something similar to matplotlib.pyplot.arrow, but I can't find anything.

Is the only way to do it using matplotlib.patches with draw_artist or something similar? I would hope that there is a simpler method, but the documentation doesn't offer much help.

Answer 1

The matplotlib ellipse demo is nice. But I could not implement it in my code without a for loop. I was getting an axes figure error. Here is what I did instead, where of course the xy center are my own coordinates with respective width and height based on the image over which I plotted the ellipse.

from matplotlib.patches import Ellipse  plt.figure() ax = plt.gca()  ellipse = Ellipse(xy=(157.18, 68.4705), width=0.036, height=0.012,                          edgecolor='r', fc='None', lw=2) ax.add_patch(ellipse) 

This code is based partially on the very first code box on this page. See Chris's response above for a link to matplotlib.patches.Ellipse.

Answer 2

If you do not want to use a patch, you can use the parametric equation of an ellipse:

x = u + a cos(t) ; y = v + b sin(t)

import numpy as np from matplotlib import pyplot as plt from math import pi  u=1.     #x-position of the center v=0.5    #y-position of the center a=2.     #radius on the x-axis b=1.5    #radius on the y-axis  t = np.linspace(0, 2*pi, 100) plt.plot( u+a*np.cos(t) , v+b*np.sin(t) ) plt.grid(color='lightgray',linestyle='--') plt.show() 

Which gives:

The ellipse can be rotated thanks to a 2D-rotation matrix :

import numpy as np from matplotlib import pyplot as plt from math import pi, cos, sin  u=1.       #x-position of the center v=0.5      #y-position of the center a=2.       #radius on the x-axis b=1.5      #radius on the y-axis t_rot=pi/4 #rotation angle  t = np.linspace(0, 2*pi, 100) Ell = np.array([a*np.cos(t) , b*np.sin(t)])        #u,v removed to keep the same center location R_rot = np.array([[cos(t_rot) , -sin(t_rot)],[sin(t_rot) , cos(t_rot)]])        #2-D rotation matrix  Ell_rot = np.zeros((2,Ell.shape[1])) for i in range(Ell.shape[1]):     Ell_rot[:,i] = np.dot(R_rot,Ell[:,i])  plt.plot( u+Ell[0,:] , v+Ell[1,:] )     #initial ellipse plt.plot( u+Ell_rot[0,:] , v+Ell_rot[1,:],'darkorange' )    #rotated ellipse plt.grid(color='lightgray',linestyle='--') plt.show() 

Returns: