我很高兴找到了解决你问题的办法（并在此过程中学习一些新技巧）。本质上，您可以创建自己的图例处理程序对象，将所有颜色Map到年份。创建自定义图例处理程序可以通过创建任何具有legend_artist(self, legend, orig_handle, fontsize, handlebox)函数的对象来完成。有关此操作的详细信息，请参阅“实现自定义处理程序”this page的部分。我在代码中注解了所有的解释，因为有太多的东西需要用语言来解释，而没有代码来演示。
示例代码：

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()

样品输出：

另一种对我有效的方法是绘制圆（椭圆-参见为什么here）和文本：

import matplotlib.patches as mpatches
#Set ellipse dimension coordinates
xmax_el=xmax/30
ymax_el=ymax/28
#Set ellipse y-location coordinates
yloc1=max(ind)+2.5
yloc2=max(ind)+1.75

#Create first circle in grey as just one grey circle is needed:
circlex=mpatches.Ellipse((pmax-.2*pmax,yloc1), xmax_el, ymax_el ,alpha=0.5,clip_on=False\
,edgecolor='grey',linewidth=2,facecolor='none')

#Start a list of patches (circles), with the grey one being the first:
patches=[circlex]

clrs=['g','r','b']
#Populate a list of circles, one for each unique color for patch names
circles=[]
for i in np.arange(len(clrs)):
   circles.append('circle'+str(i))

#This list is for multiplying by the x-position dimension to space out the colored bubbles: 
clrnum=np.arange(len(clrs))

#Reverse the order of colors so they plot in order on the chart (if clrs was based on real data that is being plotted)
clrs2=clrs[::-1]

#Iterate through the color, circle, and circle number lists, create patches, and plot.
 for i,j,k in zip(clrs2,circles,clrnum):
     j=mpatches.Ellipse((pmax-(.2+k*0.05)*pmax,yloc2),xmax_el,ymax_el,alpha=0.5,clip_on=False,edgecolor=i,linewidth=2,facecolor='none')
     patches.append(j)
for i in patches:
   ax.add_artist(i)

#Add text:
ax.text(pmax-.15*pmax,yloc1,'2015 Plan Offering',color='grey',ha='left',va='center')
ax.text(pmax-.15*pmax,yloc2,'2016 Plan Offering',color='grey',ha='left',va='center')

结果：x1c 0d1x