Adjust auto accuracy heuristic

[davidchall]

Fixes #279

The auto accuracy heuristic shows 2 significant figures, relative to the smallest difference between breaks. This is designed to handle situations like (taken from tidyverse blog post):

However, this heuristic displays an unnecessary digit in the specific case when all the breaks align with zeros in the final digit. This situation naturally occurs very often. Here's an example (taken from the scales docs):

This PR reduces the precision when final digit is always zero.

Before

library(scales)

number(seq(0, 0.1, 0.025))
#> [1] "0.000" "0.025" "0.050" "0.075" "0.100"
number(seq(0, 20, 5))
#> [1] "0.0"  "5.0"  "10.0" "15.0" "20.0"

^{Created on 2021-03-27 by the reprex package (v1.0.0)}

After

library(scales)

number(seq(0, 0.1, 0.025))
#> [1] "0.000" "0.025" "0.050" "0.075" "0.100"
number(seq(0, 20, 5))
#> [1] "0"  "5"  "10" "15" "20"

^{Created on 2021-03-27 by the reprex package (v1.0.0)}

[hadley]

Thanks! This is a great little improvement.