The geom_abline function has no position parameter

[pglezen]

The first code sample of 04_rttm.md has the following line:

g = g + geom_abline(position = "identity")

This won't compile in my version of R (3.2.3). The position parameter is not listed in the help file for geom_abline. But the program runs fine without it. So I recommend removing it.