[add] stacked area chart example (#10)

Author: extua

docs/examples/stacked-area-chart.md

@@ -0,0 +1,35 @@
+---
+title: Stacked area chart
+sidebar_custom_props:
+  tags: [fill]
+---
+
+This example shows how related series can be compounded into a stacked area chart using <Crossref target="fill-between" />.
+
+```example
+#let stacked-area(x, ..y) = {
+  y
+    .pos()
+    .fold(
+      ((0,) * x.len(),) , (ys-stacked, ys) => {
+        let previous = ys-stacked.last()
+        ys-stacked.push(lq.vec.add(previous, ys))
+        ys-stacked
+      }
+    )
+    .windows(2)
+    .map(((y1, y2)) => lq.fill-between(x, y1, y2: y2))
+}
+
+#let x = range(10)
+#let y1= (0, 1, 3, 9, 5, 4, 2, 2, 1, 0)
+#let y2 = (5, 3, 2, 0, 1, 2, 2, 2, 3, 2)
+#let y3 = (0, 0, 0, 0, 1, 2, 4, 5, 5, 9)
+
+#lq.diagram(
+  ..stacked-area(x, y1, y2, y3)
+)
+```
+
+Stacked area charts are useful for showing the relationship between categories of the same type in one chart,
+but beware that including too many series can harm readability.