Why in ggplot the orientation of the tick labels on the Y axis is horizontal?

ggplot2

#1

This occurred to me today.

In plain R, the labels of the ticks of the Y axis are oriented vertically towards the 12 o'clock. In ggplot the behavior is different, the labels of the ticks on the Y axis are oriented horizontally towards 3 o'clock.

The ggplot approach is more intuitive to me, however I am really interested in the design decision behind this behavior? Any reference related to this?!

I know ggplot takes in consideration the guidelines from Tufte, is this one of those?!

require(ggplot2)

# ggplot (plot2)
ggplot(mpg, aes(displ, hwy)) + 
  geom_point()

# plain R (plot1)
plot(mpg$displ, mpg$hwy)


#2

Though this isn't a source, the rule of thumb with respect to label orientation I've heard is that you're trying to accommodate your audience when possible (i.e. human heads aren't rotated at that 90° angle).

Hadley's paper, A Layered Grammar of Graphics, will probably offer more insight:
http://vita.had.co.nz/papers/layered-grammar.html

Also, Wilkinson's The Grammar of Graphics: