Annotate axis with text in matplotlib

Question

I want to annotate the axis of a plot with text like the example chart. To be specific, I would like to annotate regions of the axes with different captions (XYZ, ABC, MNO, etc. shown in red).

I generated the chart using this example (plotting barchart): http://matplotlib.org/examples/api/barchart_demo.html

Could anyone please help me draw such line as well as add text along axes? Any pointer to an example is also appreciated. I am not sure how else to articulate what I want to do here other than describing with a picture.

Solution

A quick read of the documentation will help, which can be found here http://matplotlib.org/users/annotations_intro.html. I have used the annotate function that is described in the documentation

Here is a piece of code that will do what you require for the x-axis. The most part of this code is taken from the example you gave a link to in your question.

N = 5
menMeans = (20, 35, 30, 35, 27)
menStd = (2, 3, 4, 1, 2)
ind = np.arange(N)  # the x locations for the groups
width = 0.35       # the width of the bars
fig, ax = plt.subplots()
rects1 = ax.bar(ind, menMeans, width, color='r', yerr=menStd)
womenMeans = (25, 32, 34, 20, 25)
womenStd = (3, 5, 2, 3, 3)
rects2 = ax.bar(ind + width, womenMeans, width, color='y', yerr=womenStd)

# add some text for labels, title and axes ticks
ax.set_ylabel('Scores')
ax.set_title('Scores by group and gender')
ax.set_xticks(ind + width)
ax.set_xticklabels(('G1', 'G2', 'G3', 'G4', 'G5'))
ax.legend((rects1[0], rects2[0]), ('Men', 'Women'))

######### annotating the x axis   #########
ax.annotate('', xy=(0, -2),xytext=(3,-2.09),                     #draws an arrow from one set of coordinates to the other
            arrowprops=dict(arrowstyle='<->',facecolor='red'),   #sets style of arrow and colour
            annotation_clip=False)                               #This enables the arrow to be outside of the plot

ax.annotate('xyz',xy=(1.1,-3.8),xytext=(1.3,-3.8),               #Adds another annotation for the text that you want
            annotation_clip=False)


ax.annotate('', xy=(3.1, -2),xytext=(5,-2.09),                   #Repeating for however many arrows you want under the axes
            arrowprops=dict(arrowstyle='<->',facecolor='red'),
            annotation_clip=False)

ax.annotate('abc',xy=(3.6,-3.8),xytext=(3.9,-3.8),
            annotation_clip=False)

######## Can add further annotations for the y-axis here similar to the above ########



# by changing the coorinates of the above you can repeat this for the y axis too
def autolabel(rects):
    # attach some text labels
    for rect in rects:
        height = rect.get_height()
        ax.text(rect.get_x() + rect.get_width()/2., 1.05*height,
                '%d' % int(height),
                ha='center', va='bottom')
autolabel(rects1)
autolabel(rects2)
plt.show()

This gives the image below:

You will need to reproduce this in order to do the same for the y-axis

Comments

[CGFoX]

Hm, trying this, it works for text, but I can't see the arrows under the horizontal axis. Do I need the autolabel function? What's the facecolor='red' for? I don't see any red there.

[Rob Romijnders]

@CGFoX, in newer versions of pyplot, the arrowstyles are specified with words, not tokens. See matplotlib.org/3.2.2/tutorials/text/…. Setting arrowstyle="simple" worked for me.