How do I display labels from both axes in a single legend when using `twinx()` in Matplotlib?

Adding a Secondary Axis Legend to a Twinx() Plot

In a plot with two y-axes created using twinx(), adding labels to the lines and displaying them in a legend can be challenging. Initially, only the labels from the primary axis may appear in the legend.

To resolve this issue, add a legend for the secondary axis using the line:

ax2.legend(loc=0)

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

However, if you want all labels on a single legend, follow these steps:

1. Define both axes and plot the lines:

   ax = fig.add_subplot(111)
   ax2 = ax.twinx()
   
   lns1 = ax.plot(...)
   lns2 = ax.plot(...)
   lns3 = ax2.plot(...)

2. Define the legend handles and labels:

   lns = lns1 + lns2 + lns3
   labs = [l.get_label() for l in lns]

3. Add the legend using:

   ax.legend(lns, labs, loc=0)

This approach will display all axis labels in a single legend.

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