Stacked area

[grantmcdermott]

Closes #632

Examples for the updated ?type_area docs.

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

#
## Stacked area plots

# Grouped area plots can be stacked cumulatively, rather than being drawn
# from a common zero baseline.

# Group B is small and steady; A and C are larger and wobblier.
dat = expand.grid(year = 2000:2020, grp = factor(c("A", "B", "C")))
dat$val = as.integer(dat$grp) +
  c(1.2, 0.1, 1.8)[dat$grp] * sin(dat$year / 3) +
  c(0.06, 0.02, 0.10)[dat$grp] * (dat$year - 2000)

tinyplot(val ~ year | grp, data = dat, type = type_area(stack = TRUE))

# Use `byord` to control which group stacks where. Here we stack by their
# largest end value.

tinyplot(
  val ~ year | grp, data = dat,
  type = type_area(stack = TRUE, byord = "end")
)

# `"minvar"` instead puts the *least variable* group on the baseline. Every
# band inherits the movement of the ones below it, so a steady bottom layer
# keeps the whole chart legible. Here that picks group B, which the default
# level order leaves in the middle and `"end"`/`"total"` push to the top.

tinyplot(
  val ~ year | grp, data = dat,
  type = type_area(stack = TRUE, byord = "minvar")
)

# `"rev"` simply flips the existing level order, which is the one thing a
# ranking function cannot do (it never sees which group it was handed).

tinyplot(
  val ~ year | grp, data = dat,
  type = type_area(stack = TRUE, byord = "rev")
)

# Custom ranking functions are also accepted. Name an argument `x` and it
# receives the group's x values too, which is what a slope needs.

tinyplot(
  val ~ year | grp, data = dat,
  type = type_area(stack = TRUE, byord = function(y, x) coef(lm(y ~ x))[2])
)

# Stacking expects a single `y` value per group per `x` value. Any repeats
# are collapsed for us first, using `FUN` (`mean` by default). Here, for
# instance, ChickWeight records many chicks per diet at each timepoint.

tinyplot(
  weight ~ Time | Diet, data = ChickWeight,
  type = type_area(stack = TRUE, FUN = median)
)

# (Illustrative purposes aside, we leave it to the reader to decide whether
# stacking separate diets on top of one another makes any sense...)

^{Created on 2026-08-23 with reprex v2.1.1}

[Copilot]

🟡 Changes recommended

Facet completion, ordered-factor handling, and legend reversal contain correctness issues.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds stacked area plots to type_area(), addressing #632.

Changes:

• Adds stacking, group ordering, aggregation, and categorical-axis support.
• Reverses stacked-area legends to match geometry.
• Adds documentation and snapshot coverage.

File summaries

File	Description
vignettes/types.qmd	Documents the legend hint.
R/type_ribbon.R	Documents stacked-area options.
R/type_area.R	Implements aggregation and stacking.
R/sanitize_bylevels.R	Adds group-order sanitization.
R/legend.R	Adds legend-key reversal.
R/assertions.R	Registers the new type hint.
NEWS.md	Announces the feature and axis fix.
man/type_ribbon.Rd	Updates generated reference documentation.
inst/tinytest/test-type_area.R	Adds area-plot snapshot tests.
inst/tinytest/_tinysnapshot/area_stack.svg	Captures default stacking.
inst/tinytest/_tinysnapshot/area_stack_flip.svg	Captures flipped stacking.
inst/tinytest/_tinysnapshot/area_stack_facet.svg	Captures faceted stacking.
inst/tinytest/_tinysnapshot/area_stack_bylevels_end.svg	Captures reordered stacking.
inst/tinytest/_tinysnapshot/area_stack_bylevels_aggregated.svg	Captures aggregated ordering.
inst/tinytest/_tinysnapshot/area_stack_alpha.svg	Captures explicit transparency.
inst/tinytest/_tinysnapshot/area_grouped.svg	Captures unstacked grouped areas.
inst/tinytest/_tinysnapshot/area_factor_x.svg	Captures categorical-axis labels.

Review details

Files not reviewed (1)

• man/type_ribbon.Rd: Generated file

Suppressed comments (2)

R/sanitize_bylevels.R:73

• The data-dependent path also recreates by as an unordered factor, causing ordered groups to switch from the sequential palette to a qualitative one whenever "start", "end", "total", or a function is used. Carry the original ordered flag into the new factor.

  factor(by, levels = levels(by)[ord])

R/legend.R:755

• Reversing only atomic values whose length exactly equals n can detach keys from labels. Expression/plotmath labels are non-atomic and remain in their original order while colours reverse; recycled aesthetics also fail (for example, two fill colours across four groups need their displayed sequence reversed). Reverse expression labels too, and expand positional aesthetics to the displayed group count before reversing.

    if (is.atomic(val) && length(val) == n) legend_args[[key]] = rev(val)

• Files reviewed: 8/17 changed files
• Comments generated: 4
• Review effort level: Balanced

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