create legend manually in matplotlib

问题

I have the following code to plot a bar graph - there are 6 species but the code groups them into three coloured groupings.

I then want to make a legend of the three groups with three colours. However, the legend only features the first grouping (which is colored correctly!) and then ignores the other groupings (see image)

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

#create data frame
G = pd.DataFrame(np.random.normal(100,20,size=(30, 1)), columns=list('G'))
T = []
for i in range(1,7):
    for j in range(5):
        T.append('Species'+repr(i))
T = pd.DataFrame(np.array(T).reshape(30,1), columns = list("T"))
cols = ['g','b','k']
C=[]
for i in range(3):
    for j in range(10):
        C.append(cols[i])
C = pd.DataFrame(np.array(C).reshape(30,1), columns = list("C"))
dat = pd.concat([G, C, T], axis = 1)
dat.columns = ['growth', 'cols', 'sp']

#begin code to make plot
grps = sorted(list(set(list(dat['sp']))))
meanGrs=[]
sems=[]
cols=[]
y_pos = np.arange(len(grps))
for grp in grps:
    qw = 'sp == "' + grp + '"'
    meanGrs.append(dat.query(qw)['growth'].mean())
    sems.append(dat.query(qw)['growth'].sem())
    cols.append(str(dat.query(qw)['cols'].max()))

plt.bar(y_pos, meanGrs, yerr=sems, align = 'center', alpha = 0.5, color = cols, label = cols)
plt.legend(('group 1', 'group 2', 'group 3'))
plt.xticks(y_pos, grps, ha = 'right', rotation = 45)
plt.show()

Any help much appreciated

回答1:

You can try this way :

for _ in dat['sp']: #looping here keeps the order
if _ not in grps:
    grps.append(_)

i=0
for grp in grps:
    dat.loc[dat['sp']==grp,'meanGrs']= dat.loc[dat['sp']==grp,'growth'].mean()
    dat.loc[dat['sp']==grp,'sems']= dat.loc[dat['sp']==grp,'growth'].sem()
    plt.bar(i, dat.loc[dat['sp']==grp,'meanGrs'], 
              yerr=dat.loc[dat['sp']==grp,'sems'], align = 'center', 
              alpha = 0.5, color = dat.loc[dat['sp']==grp,'cols'])
    i=i+1
    plt.legend(('group 1', 'group 2', 'group 3', 'group 4'))
    plt.show()

回答2:

This does the job:

import matplotlib.patches as mpatches

l1 = mpatches.Patch(color='k', label='group 1')
l2 = mpatches.Patch(color='b', label='group 2')
l3 = mpatches.Patch(color='g', label='group 3')
plt.legend(handles=[l1, l2, l3)

回答3:

Put the argument "label" in the plt.bar:

plt.bar(y_pos, meanGrs, yerr=sems, align = 'center', alpha = 0.5, color = cols, label=cols)

回答4:

I'm not quite sure what would determine which group is which in this case. But if that is arbitrary you can just create the groups on the fly.

legenditems = [(plt.Rectangle((0,0),1,1, color=c, alpha=0.5), "group "+str(i+1))
                 for i,c in enumerate(np.unique(cols))]
plt.legend(*zip(*legenditems))

来源：https://stackoverflow.com/questions/50830680/create-legend-manually-in-matplotlib

标签

python

matplotlib

legend