docs: make scales diverge using a midpoint and rescale

[davidhodge931]

10. Centre a diverging colour scale

Sometimes you need to centre a diverging colour scale.

mpg |>
  mutate(centred = cty - mean(cty)) |>
  select(displ, hwy, centred) %>%
  gg_point(
    x = displ,
    y = hwy,
    col = centred,
    col_pal = c(teal, "#E8EFF2", orange),
    col_limits = max(abs(.$centred)) * c(-1, 1),
  )

mpg |>
  mutate(centred = cty - mean(cty)) |>
  select(displ, hwy, centred) %>%
  gg_point(
    x = displ,
    y = hwy,
    col = centred,
    col_pal = c(teal, "#E8EFF2", orange),
    col_rescale = scales::rescale(sort(c(range(.$centred), 0)))
  )