Python: displaying a line of text outside a matplotlib chart

Question

I have a matrix plot produced by the matplotlib library. The size of my matrix is 256x256, and I already have a legend and a colorbar with proper ticks. I cannot attach any image due to my being new to stackoverflow. Anyhow, I use this code to generate the plot:

# Plotting - Showing interpolation of randomization
plt.imshow(M[-257:,-257:].T, origin='lower',interpolation='nearest',cmap='Blues', norm=mc.Normalize(vmin=0,vmax=M.max()))
title_string=('fBm: Inverse FFT on Spectral Synthesis')
subtitle_string=('Lattice size: 256x256 | H=0.8 | dim(f)=1.2 | Ref: Saupe, 1988 | Event: 50 mm/h, 15 min')
plt.suptitle(title_string, y=0.99, fontsize=17)
plt.title(subtitle_string, fontsize=9)
plt.show()

# Makes a custom list of tick mark intervals for color bar (assumes minimum is always zero)
numberOfTicks = 5
ticksListIncrement = M.max()/(numberOfTicks)
ticksList = []
for i in range((numberOfTicks+1)):
    ticksList.append(ticksListIncrement * i) 

cb=plt.colorbar(orientation='horizontal', format='%0.2f', ticks=ticksList) 
cb.set_label('Water depth [m]') 
plt.show()
plt.xlim(0, 255)
plt.xlabel('Easting (Cells)') 
plt.ylim(255, 0)
plt.ylabel('Northing (Cells)')

Now, being my subtitle too long (3rd line of code in the excerpt reported here), it interferes with the Y axis ticks, and I don't want this. Instead, some of the information reported in the subtitle I would like to be re-routed to a line of text to be placed at the bottom center of the image, under the colorbar label. How can this be done with matplotlib?

Sorry for not being able to attach an image. Thanks.

Answer 1

Typically, you'd use annotate to do this.

The key is to place the text with the x-coordinates in axes coordinates (so it's aligned with the axes) and the y-coordinates in figure coordinates (so it's at the bottom of the figure) and then add an offset in points so it's not at the exact bottom of the figure.

As a complete example (I'm also showing an example of using the extent kwarg with imshow just in case you weren't aware of it):

import numpy as np
import matplotlib.pyplot as plt

data = np.random.random((10, 10))

fig, ax = plt.subplots()
im = ax.imshow(data, interpolation='nearest', cmap='gist_earth', aspect='auto',
               extent=[220, 2000, 3000, 330])

ax.invert_yaxis()
ax.set(xlabel='Easting (m)', ylabel='Northing (m)', title='This is a title')
fig.colorbar(im, orientation='horizontal').set_label('Water Depth (m)')

# Now let's add your additional information
ax.annotate('...Additional information...',
            xy=(0.5, 0), xytext=(0, 10),
            xycoords=('axes fraction', 'figure fraction'),
            textcoords='offset points',
            size=14, ha='center', va='bottom')


plt.show()

Most of this is reproducing something similar to your example. The key is the annotate call.

Annotate is most commonly used to text at a position (xytext) relative to a point (xy) and optionally connect the text and the point with an arrow, which we'll skip here.

This is a bit complex, so let's break it down:

ax.annotate('...Additional information...',  # Your string

            # The point that we'll place the text in relation to 
            xy=(0.5, 0), 
            # Interpret the x as axes coords, and the y as figure coords
            xycoords=('axes fraction', 'figure fraction'),

            # The distance from the point that the text will be at
            xytext=(0, 10),  
            # Interpret `xytext` as an offset in points...
            textcoords='offset points',

            # Any other text parameters we'd like
            size=14, ha='center', va='bottom')

Hopefully that helps. The Annotation guides (intro and detailed) in the documentation are quite useful as further reading.