Hello everybody out there using ggplot,
In order to indicate a significant difference between two groups in a plot, a line and asterisks are often drawn between the groups. While this is rather easy in base graphics, the situation becomes more difficult when using ggplot. For indicating significant differences across facets, there has already been asked a related question before. I get the basic idea there, but I'm wondering whether there is a easier way in general and especially a more transparent way to get the coordinates right.
Moreover, I would be very interested in finding a good tutorial on using gtable. The answer to the question above relies on gtable, but the manual is rather short for a beginner.
