How to Add Labels for Both Primary and Secondary Axes in a Legend with TwinX?

Legend Display with Secondary Axis in TwinX

In a plot with multiple y-axes using twinx(), adding labels to each line and displaying them in a legend can present a challenge. Typically, only labels from the primary axis appear in the legend.

Consider the following example where labels for two primary axis lines and one secondary axis line are defined:

<code class="python">fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(time, Swdown, '-', label = 'Swdown')
ax.plot(time, Rn, '-', label = 'Rn')
ax2 = ax.twinx()
ax2.plot(time, temp, '-r', label = 'temp')
ax.legend(loc=0)</code>

In this case, the legend shows only the labels 'Swdown' and 'Rn'. To include the label 'temp' for the secondary axis, two approaches can be employed:

Separate Legends

One option is to create a second legend specifically for the secondary axis. This can be achieved by adding the following line:

<code class="python">ax2.legend(loc=0)</code>

This will result in two separate legends, one for each axis.

Combined Legend

For a single, combined legend, use the following steps:

1. Create a list of all the lines (from both axes) you want to appear in the legend:

<code class="python">lns = lns1+lns2+lns3</code>

2. Extract the labels from each line:

<code class="python">labs = [l.get_label() for l in lns]</code>

3. Use the legend() function on the primary axis (ax), passing in the combined line list and label list:

<code class="python">ax.legend(lns, labs, loc=0)</code>

By following these instructions, you can effectively display all line labels in a single legend, whether they belong to the primary or secondary axes.

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