breaks_pretty() will return zero-range limit

[teunbrand]

This PR aims to fix #446.

Briefly, it ensures that breaks_pretty() with zero-range x returns the one limit.

In more detail, the precedence of #446 is as follows:

transform_time()$breaks is breaks_pretty().
breaks_pretty() uses pretty()
pretty.Date() uses grDevices:::prettyDate()

The only time {scales} can intercept is during breaks_pretty(). I've tried passing different options to pretty() to preserve some formatting advantages: they don't work. I've settled on just returning the limit because it is not unreasonable to assume that if you want to label one and only one thing, the label should be at the thing you want rather than at some 'pretty' location you don't have data for.

Reprex from issue:

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales
limits <- lubridate::as_datetime("2010-08-03 00:50:50")
limits <- c(limits, limits)

trans  <- transform_time()
breaks <- trans$breaks(limits)
labels <- trans$format(breaks)
labels
#> [1] "2010-08-03 00:50:50"

^{Created on 2024-10-07 with reprex v2.1.1}

[thomasp85]

Thanks