How to plot without extending axis limits?

Question

I'm trying to draw on an existing axis without extending or modifying its limits.

For example:

import numpy as np
import matplotlib.pyplot as plt

xy = np.random.randn(100, 2)

plt.scatter(xy[:,0], xy[:,1])

Makes a fine plot with well-fitting axis limits.

However, when I try to draw a line on top of it:

xlim = plt.gca().get_xlim()
plt.plot(xlim, xlim, 'k--')

the axis limits are extended, presumably to create padding around the new data.

How can I draw a line without this padding?

Answer 1

Setting plt.autoscale(False) prevents autoscaling from happening.

import numpy as np; np.random.seed(42)
import matplotlib.pyplot as plt

xy = np.random.randn(100, 2)
# By default plots are autoscaled. 
plt.scatter(xy[:,0], xy[:,1])

#Turn autoscaling off
plt.autoscale(False)
xlim = plt.gca().get_xlim()
plt.plot(xlim, xlim, 'k--')

plt.show()