Insets

[has2k1]

Think about insets plots.

Potential syntax

# 1.
p_inset = ggplot() + ...
p = ggplot() + ... + inset(p_inset)

# 2.
p_inset = ggplot(inset=True) + ...
p = ggplot() + ... + p_inset

• The main plot and inset plot should have the same number of panels.
• Set a location or n locations for the inset panels.

[has2k1]

Consider #298 and #124 when tacking this. i.e. a way for the user to pass a figure.