Respect original axis order (added layers)

[grantmcdermott]

Fixes #516

This ended up being trickier than I first thought b/c of different edge cases, but I think it's working well now. The core idea is pretty simple, though: We capture the x/ylab variables of an original plot layer and store them in our internal (tinyplot) environment. Then we match those up against any added layers that follow.

Examples

1. Errorbar (original layer) + ribbon (secondary layer)

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot
library(broom)

coefs = tidy(lm(mpg ~ wt + drat, mtcars), conf.int = TRUE)

plt(
  estimate ~ term,
  ymin = conf.low, ymax = conf.high,
  data = coefs,
  type = "errorbar",
  theme = "basic"
)
plt_add(type = 'ribbon')

2. Reverse case: Ribbon (original layer) + errorbar (secondary layer)

plt(
  estimate ~ term,
  ymin = conf.low, ymax = conf.high,
  data = coefs,
  type = "ribbon",
  theme = "basic"
)
plt_add(type = 'errorbar')

3. Flipped example: Error bar (original layer) + line (secondary layer)

plt(
  estimate ~ term,
  ymin = conf.low, ymax = conf.high,
  data = coefs,
  type = "errorbar",
  theme = "basic",
  flip = TRUE
)
plt_add(type = 'l')

^{Created on 2025-11-20 with reprex v2.1.1}