fix: labs(alt:) fills in plot alt text (#57)

Author: mcanouil

CHANGELOG.md

@@ -12,6 +12,7 @@
 - feat: `labs()` fields default to `auto`; pass `none` to suppress an axis or legend title and reclaim the space it reserved. (#12)
 - feat: `element-blank()` on a text surface (axis, plot, or legend title) collapses the space the text would reserve. (#12)
 - feat: `width`/`height` accept `auto` to fill the available space of a bounded container. (#10)
+- fix: `labs(alt:)` fills in the figure's accessibility alt text when `plot(alt:)` is unset, instead of being stored and ignored. (#57)
 - fix: `geom-qq()`/`geom-qq-line()` honour the `distribution` argument (`uniform`/`exponential`) instead of always plotting against the normal reference. (#56)
 - fix: the `font` set on `element-text`/`element-typst`/`element-geom` is applied to every text surface and to the text-drawing geoms, inheriting the base `text` font when unset. (#54)
 - fix: a `stage`/`after-scale` channel now trains its scale on the marker's source column, so the closure receives the channel's scale-resolved value as documented. (#52)

src/plot.typ

@@ -1,6 +1,18 @@
 #import "render.typ": render-plot
 #import "data.typ": _normalise-data
 
+// Effective alt text: an explicit `plot(alt:)` wins; otherwise a `labs(alt:)`
+// fills in (its `auto` default and `none` both count as unset), so the labs
+// field reaches `get-alt-text` instead of being dropped.
+#let _resolve-alt(alt, labs) = {
+  if alt != none { return alt }
+  if labs != none {
+    let labs-alt = labs.at("alt", default: auto)
+    if labs-alt != auto and labs-alt != none { return labs-alt }
+  }
+  none
+}
+
 /// Compose a layered plot from data, aesthetics, and geom layers.
 ///
 /// `plot` is the entry point of the grammar: it resolves the dataset, wires up
@@ -104,6 +116,7 @@
   strict: false,
   defer: false,
 ) = {
+  let alt = _resolve-alt(alt, labs)
   // Deferred plots skip the context block because `context` returns
   // content; compose() resolves the active theme from its own context
   // before handing the spec to the renderer.

tests/unit/test-labs-alt.typ

@@ -0,0 +1,20 @@
+// `_resolve-alt`: plot(alt:) wins, else labs(alt:) fills in.
+
+#import "../../src/plot.typ": _resolve-alt
+#import "../../src/labs.typ": labs
+
+// --- explicit plot alt wins over a labs alt -------------------------------
+
+#assert.eq(_resolve-alt("explicit", labs(alt: "from labs")), "explicit")
+
+// --- labs alt fills in when plot alt is unset -----------------------------
+
+#assert.eq(_resolve-alt(none, labs(alt: "from labs")), "from labs")
+
+// --- both unset resolves to none ------------------------------------------
+// labs(alt:) defaults to `auto`, which counts as unset.
+
+#assert.eq(_resolve-alt(none, labs(title: "t")), none)
+#assert.eq(_resolve-alt(none, none), none)
+
+Labs alt tests passed.