[FIXED] Legend with Different Number and Color of Markers per Handle

Issue

Given the following:

import pandas as pd
import matplotlib.pyplot as plt
d=pd.DataFrame({'category':['a','a','a','b','b','b'],
                'year':[1,2,1,2,1,2],
                'x':[2,4,5,1,2,3],
                'y':[1,2,3,2,4,6],
                'clr':['grey','green','grey','blue','grey','orange']})
d
    category    clr     x   y   year
0         a     grey    2   1   1
1         a     green   4   2   2
2         a     grey    5   3   1
3         b     blue    1   2   2
4         b     grey    2   4   1
5         b     orange  3   6   2

and

for i in np.arange(len(d)):
    plt.plot(d.x[i],d.y[i],marker='o',linestyle='none',markerfacecolor=d.clr[i],
             markeredgecolor='none',markersize=15)
#clean up axes
plt.tick_params(axis='x',which='both',bottom='off',top='off',color='none',labelcolor='none')
plt.tick_params(axis='y',which='both',left='off',right='off',color='none',labelcolor='none')
lgnd=plt.legend(['Year 1','Year 2'],
            numpoints=1,
            loc=0,
            ncol=1,
            fontsize=10,
            frameon=False)
lgnd.legendHandles[0]._legmarker.set_markersize(15)
lgnd.legendHandles[1]._legmarker.set_markersize(15)

I'd like for the legend to have one grey dot for the Year 1 marker (as it currently does) but for the Year 2 markers, one dot for each distinct color (in this case, an orange, blue, and green dot all on the same line order doesn't matter at this time, in a row). Like this:

I've tried the following, but to no avail:

lgnd.legendHandles[1]._legmarker.set_numpoints(len(d.clr.unique()))
lgnd.legendHandles[1]._legmarker.set_markeredgecolor(d.clr)

Thanks in advance!

Solution

I had fun figuring out a solution to your problem (and learning a few new tricks in the process). Essentially, you could make your own legend handler object to map all colours to a year. Making a custom legend handler can be done by making any object that has function legend_artist(self, legend, orig_handle, fontsize, handlebox). The detail of why this works can be found in the "Implementing custom handler" section of this page. I commented all the explanation in the code since there is too much to explain by words without codes to demonstrate.

Example code:

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import pdb

import matplotlib.patches as mpatches


class MyLegendHandler(object):
    def __init__(self,color):
        self.color = color

    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        x0, y0 = handlebox.xdescent, handlebox.ydescent #offset of the lower left corner
        width, height = handlebox.width, handlebox.height #width, height bound box of legend, for now, it is the dimension of each circle legend
        #NOTE: to be practicle, let's just set radius = height as if width != height, it's an ellipse
        #NOTE: these will latter on be changed internally to accomdate adding text
        handlebox.width += len(self.color)*height  # reset width of handlebox to accomodate multiple legends
        for i in range(len(self.color)): #loop through all colors
            #for each color, draw a circle of that color
            #NOTE: play around here to align the legends right rather than left :)
        center = [0.5*(i + 1) * width - 0.5*x0, 0.5 * height - 0.5 * y0]
            patch = mpatches.Ellipse(center, height, height, facecolor=self.color[i],
                                   edgecolor=None, hatch=None, transform=handlebox.get_transform())

            handlebox.add_artist(patch)
        return patch


###################################

d=pd.DataFrame({'category':['a','a','a','b','b','b'],
                'year':[1,2,1,2,1,2],
                'x':[2,4,5,1,2,3],
                'y':[1,2,3,2,4,6],
                'clr':['grey','green','grey','blue','grey','orange']})
unique_year_elements = []
years_seen = []
tmp = None
my_map = {}
for i in np.arange(len(d)):
    tmp, = plt.plot(d.x[i],d.y[i],marker='o',linestyle='none',markerfacecolor=d.clr[i],
             markeredgecolor='none',markersize=15) 
    #collect the plot elements that are of unique years-- 1 year might have several plot element, we only need 1
    if not (d.year[i] in years_seen):
        years_seen.append(d.year[i])
        unique_year_elements.append(tmp)

#build handler_map for plt.legend to map elements to its legend handler object
for i in np.arange(len(years_seen)):
    color_list = d.loc[d['year'] == years_seen[i]].clr.unique().tolist()
    #pdb.set_trace()
    my_map[unique_year_elements[i]] = MyLegendHandler(color_list)

#creating the legend object
plt.legend( unique_year_elements, ["Year "+str(y) for y in years_seen],
           handler_map=my_map)
#clean up axes
plt.tick_params(axis='x',which='both',bottom='off',top='off',color='none',labelcolor='none')
plt.tick_params(axis='y',which='both',left='off',right='off',color='none',labelcolor='none')

plt.show()

Sample output:

Answered By - TuanDT