How to add a second legend for subgroups in matplotlib scatterplot

Question

I am making a plot using matplotlib which uses a colormap to show different colors for each subgroup within the plot. However for plotting purposes the subgroups are all one set of x/y pairs.

plt.scatter(rs1.x,rs1.y, marker = 'D', color=cmap ,label='data')
plt.plot(rs1.x,rs1.hub_results.predict(), marker = 'x', color = 'g',label = 'Huber Fit')
plt.plot(rs1.ol_x,rs1.ol_y, marker = 'x', color='r', ms=10, mew=2, linestyle = ' ', label='Outliers')

It gives the image shown below. It is giving me the colors as I mapped them so that part is working fine, but I have not been able to figure out how to add a second legend to the plot to show what the meaning of each color is. Apprecaite any guidance on this.

Thanks, Charlie

Answer 1

Below is an example of how to do this. Basically, you end up making two calls to legend. On the first call, you save the legend that is created to a variable. The second call removes the first legend you created, so afterwards you can manually add it back with the Axes.add_artist function.

import matplotlib.pyplot as plt
import numpy as np

x = np.random.uniform(-1, 1, 4)
y = np.random.uniform(-1, 1, 4)

p1, = plt.plot([1,2,3])
p2, = plt.plot([3,2,1])
l1 = plt.legend([p2, p1], ["line 2", "line 1"], loc='upper left')

p3 = plt.scatter(x[0:2], y[0:2], marker = 'D', color='r')
p4 = plt.scatter(x[2:], y[2:], marker = 'D', color='g')

# This removes l1 from the axes.
plt.legend([p3, p4], ['label', 'label1'], loc='lower right', scatterpoints=1)
# Add l1 as a separate artist to the axes
plt.gca().add_artist(l1)