Get ggplot plot panel limits

I have added a new function (get_plot_limits) to my package QsRutils.  It extracts the minimum and maximum X and Y values for a ggplot panel. This is useful in formatting ggplots. For example, you may wish to expand the panel to avoid text running out of the panel, or nudge text relative to some point. For an example, see my post on adding compact letter displays to box plots created with ggplot2.