How to add legend in overlapping histogram in python

Question

I'm trying to add a color legend for the overlap. I don't have a plot for this because it's the overlap of x and y so I can't add legend from label like I did for x and y ... i want to have a third line in the legend the same than x and y in purple and written overlap. thanks for your help.

X = [3,2,1,30,20,16]
Y = [4,1,6,4,34,21]
#TMM_norm(X,Y,1)
bins=[0,10,20,30,40]
plt.figure
plt.hist(X,bins=bins,edgecolor='black',color='red',alpha=0.5,label='X')
plt.hist(Y,bins=bins,edgecolor='black',color='blue',alpha=0.5,label='Y')
plt.legend()
plt.show

Here is my output:

1:

Answer 1

You will need to mix the colours and add it to the legend. Below code will do this for you. Remember to add the second color first (b2 and then b1 in code below), so that the colors come out correctly. You can edit the text as you want for the legend(X, Y, X & Y) in the last line. Code borrowed from here. Hope this helps.

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

X = [3,2,1,30,20,16]
Y = [4,1,6,4,34,21]
bins=[0,10,20,30,40]

def mix_colors(cf, cb): # cf = foreground color, cb = background color 
    a = cb[-1] + cf[-1] - cb[-1] * cf[-1] # fixed alpha calculation
    r = (cf[0] * cf[-1] + cb[0] * cb[-1] * (1 - cf[-1])) / a
    g = (cf[1] * cf[-1] + cb[1] * cb[-1] * (1 - cf[-1])) / a
    b = (cf[2] * cf[-1] + cb[2] * cb[-1] * (1 - cf[-1])) / a
    return [r,g,b,a]

b1 = [1.0, 0.0, 0.0, 0.5]
b2 = [0.0, 0.0, 1.0, 0.5]
b12  = mix_colors(b2, b1) # mix b2 over b1

plt.figure()
plt.hist(X,bins=bins,edgecolor='black',color='red',alpha=0.5)
plt.hist(Y,bins=bins,edgecolor='black',color='blue',alpha=0.5)
plt.legend()
plt.show

b1 = Line2D([0], [0], linestyle='none', marker='s', markersize=10, markerfacecolor=b1)
b2 = Line2D([0], [0], linestyle='none', marker='s', markersize=10, markerfacecolor=b2)
b12 = Line2D([0], [0], linestyle='none', marker='s', markersize=10, markerfacecolor=b12)

plt.legend((b1, b2, b12), ('X', 'Y', 'X & Y'), numpoints=1, loc='best')

Output Graph