Fix dynamic x-axis spacing

[grantmcdermott]

Fixes #368

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
tinytheme('clean', las = 2)

plt(Temp ~ I(1e5 * Month), data = airquality)

plt(Temp ~ I(month.name[Month]), data = airquality)

^{Created on 2025-04-10 with reprex v2.1.1}