def draw_ellipse(pos, cov, nsig=None, ax=None, label="temp",
set_label=True, **kwrgs):
"""
Plots an ellipse enclosing *volume* based on the specified covariance
matrix (*cov*) and location (*pos*). Additional keyword arguments are
passed on to the ellipse patch artist.
Parameters
----------
cov : The 2x2 covariance matrix to base the ellipse on
pos : The location of the center of the ellipse. Expects a 2-element
sequence of [x0, y0].
volume : The volume inside the ellipse; defaults to 0.5
ax : The axis that the ellipse will be plotted on. Defaults to the
current axis.
"""
def eigsorted(cov):
vals, vecs = np.linalg.eigh(cov)
order = vals.argsort()[::-1]
return vals[order], vecs[:, order]
if ax is None:
fig, ax = plt.subplots(1)
if nsig is None:
nsig = [1, 2]
vals, vecs = eigsorted(cov)
theta = np.degrees(np.arctan2(*vecs[:, 0][::-1]))
# Width and height are "full" widths, not radius
for ele in nsig:
scale = np.sqrt(chi2.ppf(erf(ele / np.sqrt(2)), df=2))
width, height = 2 * scale * np.sqrt(vals)
ellip = Ellipse(xy=pos, width=width, height=height, angle=theta, **kwrgs)
ax.add_artist(ellip)
ellip.set_clip_box(ax.bbox)
ellip.set_label(label)
# Limit the axes correctly to show the plots
ax.set_xlim(pos[0] - 2 * width, pos[0] + 2 * width)
ax.set_ylim(pos[1] - 2 * height, pos[1] + 2 * height)
if set_label:
ax.legend(handles=[plt.plot([], ls="-")[0]],
labels=[ellip.get_label()])
return ax
