I would like to add a rectangle to my scatter plot to specify a region. I know I can do this by calling geom_polygon and specifying the X- and Y-coordinates of the corners. However, I would like to make sure the rectangle is always half (or another fraction) of the height of the plot, regardless of what the scale of the points on the plot is.
Is there any way I can specify the coordinates of the corners with something like "fraction of plotting region" as the unit?
The only other option I see is to try to predict the scale of the plot by looking at the min() and max() of the points that I am plotting and then calculating the coordinates for the box based on that, but that becomes a lot more complicated once you start faceting and so on...