How to Position Legend Outside the Plot in Matplotlib

Introduction

In data visualization, often create complex graphs that need to have legends for the reader to be able to interpret the graph. But what if those legends get in the way of the actual data that they need to see? In this Byte, we'll see how you can move the legend so that it's outside of the plot in Matplotlib.

Legends in Matplotlib

In Matplotlib, legends provide a mapping of labels to the elements of the plot. These can be very important to help the reader understand the visualization they're looking at. Without the legend, you might not know which line represented which data! Here's a basic example of how legends work in Matplotlib:

import matplotlib.pyplot as plt

# Create a simple line plot
plt.plot([1, 2, 3, 4], [1, 4, 9, 16], label='Sample Data')

# Add a legend
plt.legend()

# Display the plot
plt.show()

This will produce a plot with a legend located in the upper-left corner inside the plot. The legend contains the label 'Sample Data' that we specified in the plt.plot() function.

Why Position the Legend Outside the Plot?

While having the legend inside the plot is the default setting in Matplotlib, it's not always the best choice. Legends can obscure important details of the plot, especially when dealing with complex data visualizations. By positioning the legend outside the plot, we can be sure that all data points are clearly visible, making our plots easier to interpret.

How to Position the Legend Outside the Plot in Matplotlib

Positioning the legend outside the plot in Matplotlib is fairly easy to do. We simply need to use the bbox_to_anchor and loc parameters of the legend() function. Here's how to do it:

import matplotlib.pyplot as plt

# Create a simple line plot
plt.plot([1, 2, 3, 4], [1, 4, 9, 16], label='Sample Data')

# Add a legend outside the plot
plt.legend(bbox_to_anchor=(1, 1.10), loc='upper right')

# Display the plot
plt.show()

In this example, bbox_to_anchor is a tuple specifying the coordinates of the legend's anchor point, and loc indicates the location of the anchor point with respect to the legend's bounding box. The coordinates are in axes fraction (i.e., from 0 to 1) relative to the size of the plot. So, (1, 1.10) positions the anchor point just outside the top right corner of the plot.

Positioning this legend is a bit more of an art than a science, so you may need to play around with the values a bit to see what works.

Common Issues and Solutions

One common issue is the legend getting cut off when you save the figure using plt.savefig(). This happens because plt.savefig() doesn't automatically adjust the figure size to accommodate the legend. To fix this, you can use the bbox_inches parameter and set it to 'tight' like so:

plt.savefig('my_plot.png', bbox_inches='tight')

Another common issue is the legend overlapping with the plot when positioned outside. This can be fixed by adjusting the plot size or the legend size to ensure they fit together nicely. Again, this is something you'll likely have to test with many different values to find the right configuration and positioning.

Note: Adjusting the plot size can be done using plt.subplots_adjust(), while the legend size can be adjusted using legend.get_frame().

Conclusion

And there you have it! In this Byte, we showed how you can position the legend outside the plot in Matplotlib and explained some common issues. We've also talked a bit about some use-cases where you'll need to position the legend outside the plot.