add code based on grid::viewport() to add rotated legend

[e-kotov]

Hi, I kindly suggest to add a short code snippet to README and one of the vignettes. This code allows to add 45-degree rotated biscale legends for better readability of labels. The easiest way to achieve this, since cowplot functions do not support rotation of inlets, is to use grid::viewport().

Preparation steps from your own readme:

library(biscale)
library(ggplot2)
library(cowplot)
library(sf)

data <- bi_class(stl_race_income, x = pctWhite, y = medInc, style = "quantile", dim = 3)

map <- ggplot() +
  geom_sf(data = data, mapping = aes(fill = bi_class), color = "white", size = 0.1, show.legend = FALSE) +
  bi_scale_fill(pal = "GrPink", dim = 3) +
  labs(
    title = "Race and Income in St. Louis, MO",
    subtitle = "Dark Blue (DkBlue) Palette"
  ) +
  bi_theme()

My addition:

library(grid)
legend <- bi_legend(pal = "GrPink",
                    dim = 3,
                    xlab = "Higher % White ",
                    ylab = "Higher Income ",
                    size = 20)
print(map)
print(legend, vp = grid::viewport(x = 0.4, y = .75, width = 0.2, height = 0.2, angle = -45))