Why is matplotlib plotting my circles as ovals?

Question

Is there a way to get matplotlib to plot a perfect circle? They look more like ovals.

Answer 1

I've encounter the same problem today and I think I might have a more flexible solution. Two main problems remain with the previous answer (if you don't use the equal aspect function). First if you resize your whole graph, the proportion will not be the same since the number of pixels will change. Second point, this trick do not work if you don't have the same lim for the xaxis and the yaxis.

This solution tricks mpl using a custom object. Indeed, whenever you change one of your axis lim or your graph size, mpl will call an internal function which will take the width and height value of the ellipse multiplied by the transform function value. Since the width and height value is stored in the ellipse object, one way is to create a custom object with a value updated whenever the function is called, based on the current ax properties :

import matplotlib.pyplot as plt
from matplotlib.patches import Ellipse

class GraphDist() :
    def __init__(self, size, ax, x=True) :
        self.size = size
        self.ax = ax
        self.x = x

    @property
    def dist_real(self) :
        x0, y0 = self.ax.transAxes.transform((0, 0)) # lower left in pixels
        x1, y1 = self.ax.transAxes.transform((1, 1)) # upper right in pixes
        value = x1 - x0 if self.x else y1 - y0
        return value

    @property
    def dist_abs(self) :
        bounds = self.ax.get_xlim() if self.x else self.ax.get_ylim()
        return bounds[0] - bounds[1]

    @property
    def value(self) :
        return (self.size / self.dist_real) * self.dist_abs

    def __mul__(self, obj) :
        return self.value * obj

fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_xlim((0,10))
ax.set_ylim((0,5))
width = GraphDist(10, ax, True)
height = GraphDist(10, ax, False)
ax.add_artist(Ellipse((1, 3), width, height))
plt.show()