Setting a fixed size for points in legend

Question

I\'m making some scatter plots and I want to set the size of the points in the legend to a fixed, equal value.

Right now I have this:

import matplotl         


        

Answer 1

You can make a Line2D object that resembles your chosen markers, except with a different marker size of your choosing, and use that to construct the legend. This is nice because it doesn't require placing an object in your axes (potentially triggering a resize event), and it doesn't require use of any hidden attributes. The only real downside is that you have to construct the legend explicitly from lists of objects and labels, but this is a well-documented matplotlib feature so it feels pretty safe to use.

from matplotlib.lines import Line2D
import matplotlib.pyplot as plt
import numpy as np

def rand_data():
    return np.random.uniform(low=0., high=1., size=(100,))

# Generate data.
x1, y1 = [rand_data() for i in range(2)]
x2, y2 = [rand_data() for i in range(2)]

plt.figure()
plt.scatter(x1, y1, marker='o', label='first', s=20., c='b')
plt.scatter(x2, y2, marker='o', label='second', s=35., c='r')

# Create dummy Line2D objects for legend
h1 = Line2D([0], [0], marker='o', markersize=np.sqrt(20), color='b', linestyle='None')
h2 = Line2D([0], [0], marker='o', markersize=np.sqrt(20), color='r', linestyle='None')

# Set axes limits
plt.gca().set_xlim(-0.2, 1.2)
plt.gca().set_ylim(-0.2, 1.2)

# Plot legend.
plt.legend([h1, h2], ['first', 'second'], loc="lower left", markerscale=2,
           scatterpoints=1, fontsize=10)
plt.show()