fix: clear error on empty manual palette

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: `scale-*-manual()` with an empty `values` array reports a clear error instead of an opaque divide-by-zero. (#36)
 - fix: number formatters carry a fraction that rounds up to a whole unit (e.g., `0.9999995` no longer renders as `0.1`), and `format-scientific` keeps its mantissa in `[1, 10)`. (#35)
 - fix: legend guides reserve space for multi-line labels, measuring the resolved custom `labels:` for both width and line count across swatch, size-ladder, and colourbar guides, so two-line content no longer clips or overlaps. (#33)
 - fix: a column mapped to both a positional aesthetic (`x`/`y`) and a grouping aesthetic (`fill`/`colour`/`group`/...), e.g., `aes(x: "class", fill: "class")`, now resolves the grouping aesthetic across aggregating stats (`boxplot`, `summary`, `count`/`sum`, histograms) instead of drawing every mark in the ink colour with an empty guide. (#31)

src/utils/palette.typ

@@ -319,7 +319,14 @@
 )
 
 // Index a palette with modulo wrap so out-of-range indices cycle.
-#let palette-at(palette, idx) = palette.at(calc.rem(idx, palette.len()))
+#let palette-at(palette, idx) = {
+  if palette.len() == 0 {
+    panic(
+      "palette-at: palette is empty; scale-*-manual() needs at least one value",
+    )
+  }
+  palette.at(calc.rem(idx, palette.len()))
+}
 
 // Resolve the palette declared on a trained scale. When the scale carries no
 // explicit palette, continuous scales receive `default-continuous`; everything