FR: lvls_revalue should accept expressions

[ashiklom]

For ggplot, I often want to make my labels use mathematical expressions, and the most straightforward way to do this is to modify the underlying factor levels.

x <- factor(c("a", "b", "c"))
my_units <- c(expression(g ~ cm^{-2}), expression(m^2), expression(mu * g ~ cm^{-2}))

This works exactly as expected:

levels(x) <- my_units

However, this currently fails with an error because my_units is not a character vector.

x <- forcats::lvls_revalue(x, my_units)

I recognize why there is value in forcing levels to be character vectors...but perhaps there could be another function (lvls_revalue2?) that is not as strict, and is basically just a pipe-friendly alias for levels<-?

[hadley]

That just turns the expressions into strings, so creating them as expressions in the first place didn't really do anything:

x <- factor(c("a", "b", "c"))
my_units <- c(expression(g ~ cm^{-2}), expression(m^2), expression(mu * g ~ cm^{-2}))
levels(x) <- my_units
levels(x)
#> [1] "g ~ cm^{\n    -2\n}"      "m^2"                     
#> [3] "mu * g ~ cm^{\n    -2\n}"

^{Created on 2019-01-04 by the reprex package (v0.2.1.9000)}