Add x=y (45 degree) line within matplotlib axis limits

Question

I have a plot with different x- and y-limits:

fig, ax = subplots(ncols=1)
ax.set_xlim([0, 10])
ax.set_ylim([5, 10])

I would like to add an x=y line to this plot, but keep the line within the axis limits.

My first, naive attempt is just

ax.plot(ax.get_xlim(), ax.get_xlim())

An improved attempt works, but is incredibly ugly, code-wise:

ax.plot([max(ax.get_xlim()[0], ax.get_ylim()[0]), 
         min(ax.get_xlim()[1], ax.get_ylim()[1])],
        [max(ax.get_xlim()[0], ax.get_ylim()[0]), 
         min(ax.get_xlim()[1], ax.get_ylim()[1])])

Is there a nicer way? I'm using IPython version 1.2.1 inside Spyder 2.2.5 on Matplotlib version 1.3.1 and mpl.get_backend() returns:

'module://IPython.kernel.zmq.pylab.backend_inline'

Answer 1

In matplotlib <= 3.2

x = np.linspace(*ax.get_xlim())
ax.plot(x, x)

In matplotlib >= 3.3

matplotlib 3.3 added the function ax.axline, which can do this better and still works if the axes limits are changed:

ax.axline([0, 0], [1, 1])

Answer 2

If 0 <= X <= 1 and 0 <= Y <= 1, this works for me:

import matplotlib.pyplot as plt

plt.scatter(X, Y)
plt.plot([0, 1], [0, 1], color = 'black', linewidth = 2)
plt.xlim(-0.05, 1.05)
plt.ylim(-0.05, 1.05)

You could adjust the limits, of course.

Answer 3

Another appraoch.

xpoints = ypoints = plt.xlim()
plt.plot(xpoints, ypoints, linestyle='--', color='k', lw=3, scalex=False, scaley=False)

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