How to Display All Labels in a Legend with twinx()?

Secondary Axis with twinx(): Enhancing Legend Display

When using twinx() to create multiple y-axes on a plot, it's desirable to display all associated labels in the legend.

Initial Issue

Consider this code snippet:

<code class="python">ax2 = ax.twinx()
ax2.plot(time, temp, '-r', label = 'temp')
ax.legend(loc=0)</code>

In this scenario, the legend only displays labels from the primary axis (ax), omitting the label for the secondary axis (ax2).

Adding a Separate Legend

To add a second legend for the secondary axis, simply include the following line:

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

This will create a separate legend for the labels associated with the secondary axis.

Combining Legends

Alternatively, to combine all labels into a single legend, follow these steps:

1. Create a list of all line objects (lns).
2. Generate a list of corresponding labels (labs).
3. Call ax.legend() using both lists to create a single legend:

<code class="python">lns = lns1 + lns2 + lns3
labs = [l.get_label() for l in lns]
ax.legend(lns, labs, loc=0)</code>

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