Substitute `iris` with another dataset, e.g. mammal sleep (ggplot2) or paperplanes

[Pakillo]

mammal sleep and paperplanes datasets have some complications (e.g. non-linear patterns) that probably advise against using them for such a very introduction to linear models...
Instead switched from iris to my simulated trees dataset (e48427c). But keeping both branches for the record (iris and trees)