Add label_log()

[davidchall]

@clauswilke - as discussed in #311. Please let me know what you think.

The default behavior of label_log() is essentially an alias for label_math(expr = 10^.x, format = log10). But it also has a base argument to support other logarithm bases (see examples below).

Since the next release already contains breaking changes, I took the liberty of removing the default expression from label_math(). I don't see any reason for this default expression over any others. But please let me know if this change should be reverted.

library(scales)

demo_log10(c(1, 1e5), labels = label_log())
#> scale_x_log10(labels = label_log())

demo_log10(c(1, 1e5), breaks = breaks_log(base = 2), labels = label_log(base = 2))
#> scale_x_log10(breaks = breaks_log(base = 2), labels = label_log(base = 2))

^{Created on 2021-06-24 by the reprex package (v2.0.0)}

[clauswilke]

I took the liberty of removing the default expression from label_math()

That's a question for @hadley, but I'd probably keep the current default it since tons of code may depend on it.

[davidchall]

I took the liberty of removing the default expression from label_math()

That's a question for @hadley, but I'd probably keep the current default it since tons of code may depend on it.

Ah, I see this function has been around for a long time. Ok - I can reverse that change.

[caparks2]

Can the concept of label_log() be extended to include formatting for other log transformations? It would be particularly helpful if it were to also work with pseudo_log_trans(). Use of label_math(expr = 10^.x, format = log10) plays havoc with the exponent values after using the pseudo log transform in the current stable release of scales.

[hadley]

I changed digits to 3 because I'd prefer not to rely on a global default (and 6 is usually going to be too many)