Center x-axis annotations by default

[thibautjombart]

The current default for x-axis annotations is to be left-justified, e.g.:

library(outbreaks)
library(incidence2)

x <- ebola_sim_clean$linelist %>%
  incidence(date_index = date_of_onset, interval = "month")
plot(x) + scale_x_incidence(x, format = "%d %M %Y")
#> Scale for 'x' is already present. Adding another scale for 'x', which will
#> replace the existing scale.

I suspect most users will want to center the labels by default. It could be done by adding a center_labels = TRUE by default to scale_x_incidence, but this argument already exists with a different meaning (position of the tick marks) in the plot method.

One solution would be to:

1. rename center_labels to center_tick in the plot method
2. add a center_labels argument to scale_x_incidence

But I am sure there are other options - just can't think of a good one atm.

[TimTaylor]

closed with the v0.2.0 release 06d70b4. I've renamed center_labels to center_ticks and fixed a bug so the labels are centrally placed and nicely spaced for facets.