Remove warning when guide functions are used in identity scales

[clauswilke]

This PR closes #3127 by adding a proper check for the presence or absence of a guide in the identity scales.

devtools::load_all("~/github/ggplot2")
#> Loading ggplot2

d <- base::data.frame(
  week = seq(1, 52, 1), 
  revenue = round(runif(52, 0, 100))
)

# discrete identity scale
ggplot(data = d, aes(x = week, y = revenue, fill = 'lightskyblue')) + 
  geom_col() + 
  scale_fill_identity(guide = guide_legend())

# continuous identity scale
ggplot(data = d, aes(x = week, y = 1, size = revenue/20)) + 
  geom_point() + 
  scale_size_identity(guide = guide_legend())

^{Created on 2019-02-11 by the reprex package (v0.2.1)}

[yutannihilation]

I prefer identical(self$guide, "none") in this case.

[clauswilke]

thanks, fixed.

[lock]

This old issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with reprex) and link to this issue. https://reprex.tidyverse.org/