Added layers should respect original x-axis order

[grantmcdermott]

On reflection, this is a separate issue (albeit related to #493):

Actually, it's not just "dodged" types. We also face a similar problem when adding on top of plot types that respect a certain order.

library(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')

^{Created on 2025-11-19 with reprex v2.1.1}

Originally posted by @grantmcdermott in #493