how to customize color legend when using for loop

Question

I want to draw a 3D scatter, in which the data is colored by group. Here is the data sample:

aa=pd.DataFrame({'a':[1,2,3,4,5],
                 'b':[2,3,4,5,6],
                 'c':[1,3,4,6,9],
                 'd':[0,0,1,2,3],
                 'e':['abc','sdf','ert','hgf','nhkm']})

Here, a, b, c are axis x, y, z. e is the text shown in the scatter. I need d to group the data and show different colors.

Here is my code:

fig = plt.figure()
ax = fig.gca(projection='3d')
zdirs = aa.loc[:,'e'].__array__()
xs = aa.loc[:,'a'].__array__()
ys = aa.loc[:,'b'].__array__()
zs = aa.loc[:,'c'].__array__()
colors = aa.loc[:,'d'].__array__()
colors1=np.where(colors==0,'grey',
                 np.where(colors==1,'yellow',
                          np.where(colors==2,'green',
                                   np.where(colors==3,'pink','red'))))
for i in range(len(zdirs)): #plot each point + it's index as text above
    ax.scatter(xs[i],ys[i],zs[i],color=colors1[i])
    ax.text(xs[i],ys[i],zs[i],  '%s' % (str(zdirs[i])), size=10, zorder=1, color='k')
ax.set_xlabel('a')
ax.set_ylabel('b')
ax.set_zlabel('c')
plt.show()

But I do not know how to put a legend on the plot. I hope my legend is like:

The colors and the numbers should match and be ordered.

Could anyone help me with how to customize the color bar?

score 2 · Accepted Answer · answered May 16 '19 at 06:36

First of all, I've taken the liberty to reduce your code a bit:

I'd suggest to create a ListedColormap to map integer->color, which allows you to pass the color column via c=aa['d'] (note it's c=, not color=!)
you don't need to use __array__() here, in the code below you can directly use aa['a']
finally, you can add an empty scatter plot for each color in the ListedColormap, and this can then be rendered correctly by ax.legend()

import pandas as pd
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
import numpy as np

from matplotlib.colors import ListedColormap
import matplotlib.patches as mpatches

aa=pd.DataFrame({'a':[1,2,3,4,5],
                 'b':[2,3,4,5,6],
                 'c':[1,3,4,6,9],
                 'd':[0,0,1,2,3],
                 'e':['abc','sdf','ert','hgf','nhkm']})

fig = plt.figure()
ax = fig.gca(projection='3d')

cmap = ListedColormap(['grey', 'yellow', 'green', 'pink','red'])
ax.scatter(aa['a'],aa['b'],aa['c'],c=aa['d'],cmap=cmap)

for x,y,z,label in zip(aa['a'],aa['b'],aa['c'],aa['e']):
    ax.text(x,y,z,label,size=10,zorder=1)

# Create a legend through an *empty* scatter plot
[ax.scatter([], [], c=cmap(i), label=str(i)) for i in range(len(aa))]
ax.legend()

ax.set_xlabel('a')
ax.set_ylabel('b')
ax.set_zlabel('c')

plt.show()

how to customize color legend when using for loop

1 Answers1