WIP: make nudge_x and nudge_y aesthetics of geom_text()

[clauswilke]

This is an example implementation of the feature I proposed in #3026 (comment). Not sure whether this is the right way to go or not.

library(ggplot2)

df <- data.frame(
  x = c(1,3,2,5),
  y = 1:4,
  label = c("a","c","d","c")
)

# new implementation, using parameters/aesthetics
p1 <- ggplot(df, aes(x, y)) +
  geom_point() +
  geom_text(aes(label = label), nudge_y = c(-0.1, 0.1, -.5, -0.1))

# current implementation, using position adjustment
p2 <- ggplot(df, aes(x, y)) +
  geom_point() +
  geom_text(aes(label = label), position = position_nudge(y = c(-0.1, 0.1, -.5, -0.1)))

# results are identical
cowplot::plot_grid(p1, p2)

cowplot::plot_grid(p1 + scale_y_log10(), p2 + scale_y_log10())

cowplot::plot_grid(p1 + coord_polar(), p2 + coord_polar())

cowplot::plot_grid(p1 + coord_trans(y = "sqrt"), p2 + coord_trans(y = "sqrt"))

# providing nudge values as aesthetics works
df <- data.frame(
  x = c(1,3,2,5),
  y = 1:4,
  nudge_y = c(-0.1, 0.1, -.5, -0.1),
  label = c("a","c","d","c")
)
ggplot(df, aes(x, y)) +
  geom_point() +
  geom_text(aes(label = label, nudge_y = nudge_y))

# incorrect length causes error as expected
ggplot(df, aes(x, y)) +
  geom_point() +
  geom_text(aes(label = label), nudge_y = c(-0.1, 0.1, -.5))
#> Error: Aesthetics must be either length 1 or the same as the data (4): nudge_y

# can specify both aesthetic and position, both nudgings are applied
ggplot(df, aes(x, y)) +
  geom_point() +
  geom_text(
    aes(label = label),
    nudge_y = c(-0.1, 0.1, -.5, -0.1),
    position = position_nudge(y = c(-0.1, 0.1, -.5, -0.1))
  )

^{Created on 2018-12-07 by the reprex package (v0.2.1)}

[clauswilke]

As discussed in #3026, this is probably not the right way to go. Instead, position adjustments should understand aesthetics. I'll close this PR.

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