Axis range for non-numeric vars

[grantmcdermott]

Fixes #511

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 = 'hline')

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

Two notes:

• The immediate solution to the above example is realizing that we don't need to calculate the axis limits again for secondary plots layers (i.e., where add = TRUE). We skip those cases now per cb6c156.
• For the more general solution—not limited to add cases—I'm coercing to numeric before getting the axis range in lim.R. This seems to work fine and doesn't break any tests; partly because we already coerce character x and y variables to factors in sanitize_data.R. But I'm flagging it as a mental touch point in case it does end up breaking something down the line.