Lunalight Lunalight - 3 years ago 334
Python Question

How to add a legend to matplotlib scatter plot

I'm attempting to plot a PCA and one of the colours is label 1 and the other should be label 2. When I want to add a legend with

ax1.legend()
I only get the label for the blue dot or no label at all. How can I add the legend with the correct labels for both the blue and purple dots?

sns.set(style = 'darkgrid')

fig, ax1 = sns.plt.subplots()

x1, x2 = X_bar[:,0], X_bar[:,1]
ax1.scatter(x1, x2, 100, edgecolors='none', c = colors)
fig.set_figheight(8)
fig.set_figwidth(15)


link to image of plot

Answer Source

It looks like you are plotting each point oscillating between two colours. As per the answer to this question subsampling every nth entry in a numpy array You can use numpys array slicing to plot two separate arrays, then do legend as normal. For some sample data:

import numpy as np
import numpy.random as nprnd
import matplotlib.pyplot as plt

A = nprnd.randint(1000, size=100)
A.shape = (50,2)

x1, x2 = np.sort(A[:,0], axis=0), np.sort(A[:,1], axis=0)

x1
Out[50]: 
array([ 46,  63,  84,  96, 118, 127, 137, 142, 181, 187, 187, 207, 210,
       238, 238, 330, 334, 335, 346, 346, 350, 392, 400, 426, 467, 531,
       550, 567, 569, 572, 583, 625, 637, 661, 671, 677, 698, 713, 777,
       796, 837, 850, 866, 868, 874, 890, 919, 972, 992, 993])
x2
Out[51]: 
array([  2,  44,  49,  51,  72,  84,  86, 118, 120, 133, 150, 155, 156,
       159, 199, 202, 250, 281, 289, 317, 317, 386, 405, 414, 427, 461,
       507, 510, 543, 552, 553, 555, 559, 576, 618, 622, 633, 647, 665,
       672, 682, 685, 745, 767, 776, 802, 808, 813, 847, 973])


labels=['blue','red']

fig, ax1 = plt.subplots()
ax1.scatter(x1[0::2], x2[0::2], 100, edgecolors='none', c='red', label = 'red')
ax1.scatter(x1[1::2], x2[1::2], 100, edgecolors='none', c='black', label = 'black')

plt.legend()
plt.show()

enter image description here

For your code, you can do:

sns.set(style = 'darkgrid')
fig, ax1 = sns.plt.subplots()  

x1, x2 = X_bar[:,0], X_bar[:,1] 
ax1.scatter(x1[0::2], x2[0::2], 100, edgecolors='none', c = colors[0], label='one')
ax1.scatter(x1[1::2], x2[1::2], 100, edgecolors='none', c = colors[1], label='two')
fig.set_figheight(8)
fig.set_figwidth(15)
plt.legend()
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