improve point size calculations

[teunbrand]

This PR aims to fix #83 and replaces #251.

Briefly, it is a more surgical approach than #251 where we only execute calculations involving the point size in centimetres and keep the rest as-is.

Reprex from the issue:

devtools::load_all("~/packages/ggrepel")
#> ℹ Loading ggrepel
#> Loading required package: ggplot2

set.seed(42)
d <- data.frame(
  x = c(1, 2, 2, 3),
  y = c(1, 2, 3, 1),
  pointsize = c(0, 2, 1, 0),
  label = sprintf("label%s", 1:4)
)

ggplot(d, aes(x, y)) +
  geom_point(aes(size = pointsize), color = "grey50") +
  geom_text_repel(
    min.segment.length = 0.1,
    box.padding = 0.2,
    aes(label = label, point.size = pointsize),
    size = 5, max.iter = 1e5, max.time = 1
  ) +
  continuous_scale(
    aesthetics = c("size", "point.size"), 
    palette = scales::area_pal(c(2, 50)), guide = "none"
  )

Example of #251's Achilles' heel:

set.seed(42)
ggplot(mtcars, aes(x = wt, y = 1, label = rownames(mtcars))) +
  geom_point(color = "red") +
  geom_text_repel(
    force_pull   = 0, # do not pull toward data points
    nudge_y      = 0.05,
    direction    = "x",
    angle        = 90,
    hjust        = 0,
    segment.size = 0.2,
    max.iter = 1e4, max.time = 1,
    point.size = 1.5
  ) +
  xlim(1, 6) +
  ylim(1, 0.8) +
  theme(
    axis.line.y  = element_blank(),
    axis.ticks.y = element_blank(),
    axis.text.y  = element_blank(),
    axis.title.y = element_blank()
  )

^{Created on 2024-12-02 with reprex v2.1.1}

[slowkow]

Thank you!! 🙏 I will run through some examples and check for any issues. Otherwise, this looks great.