Does matplotlib have a function for drawing diagonal lines in axis coordinates?

Question

Matplotlib Axes have the functions axhline and axvline for drawing horizontal or vertical lines at a given y or x coordinate (respectively) indepen

Answer 1

Plotting a diagonal line based from the bottom-left to the top-right of the screen is quite simple, you can simply use ax.plot(ax.get_xlim(), ax.get_ylim(), ls="--", c=".3"). The method ax.get_xlim() will simply return the current values of the x-axis (and similarly for the y-axis).

However, if you want to be able to zoom using your graph then it becomes slightly more tricky, as the diagonal line that you have plotted will not change to match your new xlims and ylims.

In this case you can use callbacks to check when the xlims (or ylims) have changed and change the data in your diagonal line accordingly (as shown below). I found the methods for callbacks in this example. Further information can also be found here

import numpy as np
import matplotlib.pyplot as plt

mean, cov = [0, 0], [(1, .6), (.6, 1)]
x, y = np.random.multivariate_normal(mean, cov, 100).T
y += x + 1

f, ax = plt.subplots(figsize=(6, 6))

ax.scatter(x, y, c=".3")
ax.set(xlim=(-3, 3), ylim=(-3, 3))

# Plot your initial diagonal line based on the starting
# xlims and ylims.
diag_line, = ax.plot(ax.get_xlim(), ax.get_ylim(), ls="--", c=".3")

def on_change(axes):
    # When this function is called it checks the current
    # values of xlim and ylim and modifies diag_line
    # accordingly.
    x_lims = ax.get_xlim()
    y_lims = ax.get_ylim()
    diag_line.set_data(x_lims, y_lims)

# Connect two callbacks to your axis instance.
# These will call the function "on_change" whenever
# xlim or ylim is changed.
ax.callbacks.connect('xlim_changed', on_change)
ax.callbacks.connect('ylim_changed', on_change)

plt.show()

Note that if you don't want the diagonal line to change with zooming then you simply remove everything below diag_line, = ax.plot(...