Question

I have a graph, computed from some data, drawn in matplotlib. I want to draw a rectangular region around the global maximum of this graph. I tried plt.axhspan, but the rectangle doesn’t seem to appear when I call plt.show()

So, how can a rectangular region be drawn onto a matplotlib graph? Thanks!

Answer 1

The most likely reason is that you used data units for the x arguments when calling axhspan. From the function’s docs (my emphasis):

y coords are in data units and x coords are in axes (relative 0-1)
units.

So any rectangle stretching left of 0 or right of 1 is simply drawn off-plot.

An easy alternative might be to add a Rectangle to your axis (e.g., via plt.gca and add_patch); Rectangle uses data units for both dimensions. The following would add a grey rectangle with width & height of 1 centered on (2,3):

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

fig = plt.figure()
plt.xlim(0, 10)
plt.ylim(0, 12)

someX, someY = 2, 5
currentAxis = plt.gca()
currentAxis.add_patch(Rectangle((someX - .5, someY - .5), 4, 6, facecolor="grey"))

• Without facecolor

currentAxis.add_patch(Rectangle((someX - .5, someY - .5), 4, 6, facecolor="none", ec='k', lw=2))