This should be easy to achieve by just allowing to add a new plot as a child of the already existing plot. The child plot should be freely moveable like plot's title (TextLabel).

Maybe we can also add a wizard for creating an inset plot with the data zoomed in - the user select a region on the original plot and there is somewhere an option to add an inset plot to the original plot. After this is selected, a new child plot is added together with the curves and the data regions adjusted to the previous selection.