How do I set label for an already plotted line in matplotlib?

Question

In my code I've already executed

ax.plot(x, y, 'b.-', ...)

and need to be able to set the label for the corresponding line after the fact, to have the same effect as if I'd

ax.plot(x, y, 'b.-', label='lbl', ...)

Is there a way to do this in Matplotlib?

Answer 1

If you grab the line2D object when you create it, you can set the label using line.set_label():

line, = ax.plot(x, y, 'b.-', ...)
line.set_label('line 1')

If you don't, you can find the line2D from the Axes:

ax.plot(x, y, 'b.-', ...)
ax.lines[-1].set_label('line 1')

Note, ax.lines[-1] will access the last line created, so if you make more than one line, you would need to be careful which line you set the label on using this method.

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A minimal example:

import matplotlib.pyplot as plt
fig,ax = plt.subplots(1)
l,=ax.plot(range(5))
l.set_label('line 1')
ax.legend()
plt.show()