Fix factor-based y-tick labels for "p"-alike plots

[grantmcdermott]

Fixes #386.

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
tinytheme("bw")

plt(
  species ~ flipper_len, data = penguins,
  type = "p"
)

plt(
  species ~ flipper_len, data = penguins,
  type = "j"
)

^{Created on 2025-05-07 with reprex v2.1.1}