type_abline()-alike now respect flip = TRUE

[grantmcdermott]

Fixes #733

I ended up consolidating much of the type_ab/h/v/line() code in the process. I also had to update some ancillary examples that were using the (wrong) workaround for adding straight lines to flipped plots. This may end up triggering revdep issues for other packages, so we'll need to check carefully.

MWE

Using the same motivating examples as #733...

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot
dat = data.frame(x = -1:1, y = 0:2)
plt(y ~ x, data = dat)
plt_add(type = "vline", lty = 2, flip = TRUE)

coefplot = function(mod, ...) {
  coefs = data.frame(names(coef(mod)), coef(mod), confint(mod))
  colnames(coefs) = c("term", "est", "lwr", "upr")
  plt(
    est ~ term, ymin = lwr, ymax = upr, data = coefs,
    type = "errorbar",
    theme = "classic",
    ...
  )
  plt_add(type = "hline", lty  = 4, col = "hotpink")
}
mod = lm(mpg ~ 0 + wt / factor(am), mtcars)
coefplot(mod, flip =TRUE)

^{Created on 2026-09-16 with reprex v2.1.1}

[Copilot]

🟡 Changes recommended

Unresolved moderate validation and mixed-slope transformation issues remain.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

This PR makes type_abline(), type_hline(), and type_vline() respect flipped plots through shared rendering logic.

Changes:

• Adds flip-aware line orientation and slope transformations.
• Consolidates straight-line implementations.
• Updates examples, documentation, NEWS, and snapshot coverage.

Review findings:

• R/type_abline.R — moderate (1 vote): NULL parameters can bypass validation; mixed zero/nonzero slopes are transformed incorrectly.
• R/type_abline.R — nit (1 vote): Add flipped zero-slope and grouped/faceted coverage.
• R/type_abline.R — nit (1 vote): Add active unflipped type_abline() coverage, including b = 0.
• vignettes/gallery_figs/likert.R — nit (2 votes): Update the remaining flipped Likert examples in R/type_barplot.R and generated documentation.

File summaries

File	Summary
vignettes/gallery_figs/likert.R	Updates flipped Likert examples.
R/type_vline.R	Delegates vertical-line rendering to shared logic.
R/type_hline.R	Delegates horizontal-line rendering to shared logic.
R/type_errorbar.R	Updates a flipped example.
R/type_abline.R	Consolidates line rendering and flip transformations.
NEWS.md	Documents the behavior change.
man/type_errorbar.Rd	Updates generated errorbar documentation.
man/type_abline.Rd	Updates generated line documentation.
inst/tinytest/test-type_pointrange.R	Updates a flipped layer example.
inst/tinytest/test-flip.R	Adds flipped-line snapshot coverage.
inst/tinytest/_tinysnapshot/flip_ablines.svg	Stores expected snapshot output.

Review details

Files not reviewed (2)

• man/type_abline.Rd: Generated file
• man/type_errorbar.Rd: Generated file

Suppressed comments (4)

R/type_abline.R:117

• Filtering NULL before validation drops the active argument for calls such as type_hline(h = NULL) and type_vline(v = NULL). These constructors previously called assert_numeric() on their supplied parameter, so this is a validation regression that can defer the failure to an empty abline() call. Build params from the selected line type and validate those entries before removing unused fields.

  params = params[!vapply(params, is.null, logical(1))]

R/type_abline.R:188

• With a numeric continuous by, the parameter vectors remain unsliced because the !by_continuous branches are skipped, so b can contain both zero and nonzero slopes. The all() check only handles the all-zero case; mixed vectors turn the zero-slope entries into a = -a / 0 and b = 1 / 0 rather than drawing those entries as vertical v = a lines. Split zero-slope entries and transform only the nonzero slopes, preserving the matching aesthetics for each line.

        abline = if (all(params[["b"]] == 0)) {
          list(v = params[["a"]])
        } else {
          list(a = -params[["a"]] / params[["b"]], b = 1 / params[["b"]])

R/type_abline.R:185

• The new flipped snapshot covers scalar h/v values and a nonzero-slope abline, but not this b == 0 branch. That branch is the special-case transformation for a horizontal original line and can regress independently; please add a flipped zero-slope abline test (and, ideally, a grouped/faceted parameter-vector case) so the new shared implementation's edge paths are protected.

        abline = if (all(params[["b"]] == 0)) {

R/type_abline.R:193

• The active tests still do not render an unflipped type_abline(): inst/tinytest/test-type_abline.R only checks its no-data error, while the new snapshot exercises only the flipped path. Since this refactor replaces the entire a/b drawing implementation, add an active unflipped case (including the b = 0 conversion branch) to catch regressions in the common path.

    do.call(abline, c(params, list(col = icol, lty = ilty, lwd = ilwd)))

• Files reviewed: 8/11 changed files
• Comments generated: 1
• Review effort level: Lite

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