fix: skip incomplete cells in contour grid

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: contour stats skip incomplete cells when the `(x, y, z)` grid is sparse instead of panicking. (#37)
 - 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)

src/utils/isobands.typ

@@ -71,6 +71,8 @@
       let se = z.at(i + 1).at(j)
       let ne = z.at(i + 1).at(j + 1)
       let nw = z.at(i).at(j + 1)
+      // Sparse grids leave incomplete cells as `none`; skip them.
+      if sw == none or se == none or ne == none or nw == none { continue }
       let poly = isoband-cell(xw, xe, ys-bot, yn-top, nw, ne, se, sw, lo, hi)
       if poly.len() >= 3 { out.push(poly) }
     }

src/utils/marching-squares.typ

@@ -81,6 +81,8 @@
       let se = z.at(i + 1).at(j)
       let ne = z.at(i + 1).at(j + 1)
       let nw = z.at(i).at(j + 1)
+      // Sparse grids leave incomplete cells as `none`; skip them.
+      if sw == none or se == none or ne == none or nw == none { continue }
       let case = (
         (if nw >= level { 8 } else { 0 })
           + (if ne >= level { 4 } else { 0 })