How do I add a legend to for pixmapped image

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Summary

To add a legend to a pixmapped image plotted with matplotlib’s axes interface, we need to create custom legend handles that represent each integer value in the 2D array. The built-in colorizer handles the lookup from integers to RGB values, but it does not provide a straightforward way to create a legend.

Root Cause

The root cause of this issue is that ax.imshow returns an AxesImage object, which cannot be used directly as a handle for the legend. The axis.legend() method requires handles that are artists from the plot itself.

Why This Happens in Real Systems

This issue occurs in real systems because:

  • Matplotlib’s axes interface is designed for more complex plots, where the legend is typically created from artists such as lines, scatter plots, or patches.
  • The AxesImage object returned by ax.imshow is not an artist in the classical sense, but rather a raster image that is displayed on the axes.
  • The colorizer is a separate component that maps integers to RGB values, but it does not provide a way to create a legend.

Real-World Impact

The real-world impact of this issue is:

  • Lack of clarity in the plot, as the reader may not understand the mapping between integer values and colors.
  • Difficulty in interpreting the plot, especially for readers who are not familiar with the colorizer used.
  • Limited customization options, as the AxesImage object does not provide a way to create a custom legend.

Example or Code

import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
import numpy as np

# Create a sample 2D array
data = np.random.randint(0, 5, size=(10, 10))

# Create a figure and axis
fig, ax = plt.subplots()

# Plot the data using ax.imshow
im = ax.imshow(data, cmap='viridis')

# Create custom legend handles
handles = [mpatches.Patch(color=plt.cm.viridis(i), label=f'Value {i}') for i in range(5)]

# Add the legend to the axis
ax.legend(handles=handles)

# Show the plot
plt.show()

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Creating custom legend handles using matplotlib.patches or other artist objects.
  • Using the colorizer to map integer values to RGB values, and then creating a Patch object for each value.
  • Adding the custom legend handles to the axis.legend() method to create the legend.

Why Juniors Miss It

Juniors may miss this issue because:

  • They may not be familiar with the matplotlib axes interface and the colorizer component.
  • They may not understand the difference between an AxesImage object and an artist object.
  • They may not know how to create custom legend handles using matplotlib.patches or other artist objects.

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