log_breaks Not Always Useful

[billdenney]

This is very similar to #100 but a more specialized case.

The second example below does not provide useful breaks (none show up when plotted). I like the "relevant_breaks" addition in #117, and similar code should be added to log_sub_breaks. If log_sub_breaks cannot find >= (n-2) breaks, then I think we should fall back to extended_breaks which has better guarantees of the number of relevant breaks even if we lose the equidistant log-scale spacing.

library(scales, quietly = TRUE)
(scales::log_breaks(base = exp(1)))(c(2, 2.0001))
#> [1] 1.000000 2.000000 2.718282
(scales::log_breaks(base = exp(1)))(c(2.0001, 2.0002))
#> [1] 2.000000 2.718282
(scales::extended_breaks())(c(2.0001, 2.0002))
#> [1] 2.000100 2.000125 2.000150 2.000175 2.000200

This affects many figures for me, so if possible to make it into 1.0.0, I think that would be selfishly beneficial (#157).

[dpseidel]

1.0.0 is up on CRAN as of this morning so unfortunately any change won't make that, but I'd be happy to review/discuss a PR for the dev version.

You're certainly right, currently the log_breaks code is not designed for very small ranges when even integer multiples of base are not sufficient, but my (perhaps wildly off-base) guess is that it's a fairly rare to need/want to transform data over such a small range, in which case scales::extended_breaks() is already the default for all non-log transformed scales.

[billdenney]

Congratulations on 1.0.0!

My use case is that I often generate many (hundreds to thousands) of figures automatically in both linear and semi-log scales for reports. If I didn't provide the log-scale version, I'd be asked why I don't have it.
While I could add testing for small ranges to all of these figures, it is preferable to have scales do what I'm hoping for (which is to give a usable scale regardless of the input).

PR coming shortly.