Parse Legends For a Matplotlib Figure

[umesh-timalsina]

Currently, the legends for a figure are parsed from labels (for line plots). Alternatively, we could parse legend for the axes from matplotlib and assign the labels.

[brollb]

What's the motivation for changing this? Is there an issue with the current approach?

[umesh-timalsina]

This is to replicate user program. Don't show the legend if the legend visibility is off in Matplotlib.

[umesh-timalsina]

This will be closed by #1747.