在matplotlib中为subplot添加标签

Question

Is there an automatic way to add pure labels to the subplots? To be specific, I used

ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)

and I would like to add 'A' and 'B' to the upper right in the subplots to distinguish them, and right now I am using a dummy way something like

ax1.annotate('A', xy=(2, 1), xytext=(1, 22))
ax2.annotate('B', xy=(2, 1), xytext=(1, 22))

I tried using

ax1.legend()

and that also gives me "small images" of lines or dots before the letter and I do not need that image.

Answer 1

You can use annotate , but you'll need to set the correct limits so they are in the "upper right corner". If you call the annotate commands after you've made all the plots, this should work since it pulls the limits from the axis itself.

import pylab as plt

fig = plt.figure()
ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)

def get_axis_limits(ax, scale=.9):
    return ax.get_xlim()[1]*scale, ax.get_ylim()[1]*scale

ax1.annotate('A', xy=get_axis_limits(ax1))
ax2.annotate('B', xy=get_axis_limits(ax2))
plt.show()

It's also worth looking at the other ways to put text on the figure.

Answer 2

You can skip writing a helper function and just call:

ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)

ax1.annotate("A", xy=(0.9, 0.9), xycoords="axes fraction")
ax2.annotate("B", xy=(0.9, 0.9), xycoords="axes fraction")

Answer 3

Answer by hooked works, but keep in mind that you need to scale the position properly.

def text_coords(ax=None,scalex=0.9,scaley=0.9):
  xlims = ax.get_xlim()
  ylims = ax.get_ylim()
  return {'x':scalex*np.diff(xlims)+xlims[0],
        'y':scaley*np.diff(ylims)+ylims[0]}


scalex = [0.02,0.02,0.75,0.75]
scaley = [0.02,0.75,0.02,0.75]
labels = ['(a)','(b)','(c)','(d)']

f,ax = plt.subplots(2,2)
for sx,sy,a,l in zip(scalex,scaley,np.ravel(ax),labels):
   a.text(s=l,**text_coords(ax=a,scalex=sx,scaley=sy))
plt.tight_layout()
plt.show()

labels demo