From 91873e03eb82d723b83d692e42d350490142e773 Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Mon, 12 Oct 2026 08:31:20 +0200
Subject: [PATCH] Draw data labels on area and stacked area charts (#545)

---
 crates/chart/src/render.rs |  9 +++++++++
 crates/chart/src/tests.rs  | 26 ++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/crates/chart/src/render.rs b/crates/chart/src/render.rs
index 63d30d982..f1b1b3eac 100644
--- a/crates/chart/src/render.rs
+++ b/crates/chart/src/render.rs
@@ -970,6 +970,8 @@ fn cartesian(ctx: &mut Ctx, chart: &Chart, data: &ChartData, area: Bx, horizonta
     let nslots = slot_keys.len().max(1) as f32;
     let bw = cw / (nslots + 1.5);
     let labels_on = chart.data_labels && ncat <= MAX_LABELS;
+    // Area labels wait until every area is filled, so a later (stacked) area can't paint over them.
+    let mut area_labels = Vec::new();
 
     // Areas first, then columns, then lines on top.
     for pass in 0..3 {
@@ -998,6 +1000,10 @@ fn cartesian(ctx: &mut Ctx, chart: &Chart, data: &ChartData, area: Bx, horizonta
                         let c = center(j);
                         top.push([c, vpos(t, &sc)]);
                         base.push([c, vpos(b, &sc)]);
+                        // Source values (not stack totals), just above the top edge like the line labels.
+                        if labels_on && sp.is_some() && let Some(v) = s.values.get(j).copied().flatten() {
+                            area_labels.push((c, vpos(t, &sc) - 5.0, fmt.format(v, None)));
+                        }
                     }
                     if top.len() == 1 {
                         // A single point: give it some width.
@@ -1072,6 +1078,9 @@ fn cartesian(ctx: &mut Ctx, chart: &Chart, data: &ChartData, area: Bx, horizonta
                 }
             }
         }
+        for (x, y, t) in area_labels.drain(..) {
+            ctx.text(x, y, &t, LABEL_SIZE, TEXT, HAlign::Center, VAlign::Bottom);
+        }
     }
     // Trendlines on top: x is the category number 1..n.
     let mut trend_labels = Vec::new();
diff --git a/crates/chart/src/tests.rs b/crates/chart/src/tests.rs
index a94267713..7fb830184 100644
--- a/crates/chart/src/tests.rs
+++ b/crates/chart/src/tests.rs
@@ -902,3 +902,29 @@ fn union_series_references() {
     c.series[0].values = format!("{}Sheet1!$B$1,Sheet1!$B$2{}", "(".repeat(5000), ")".repeat(5000));
     let _ = resolve(&wb, 0, &c);
 }
+
+#[test]
+fn area_charts_draw_source_value_labels_only_when_enabled() {
+    for kind in [ChartKind::Area, ChartKind::AreaStacked] {
+        let d = sample(kind);
+        let mut c = chart(kind);
+        c.data_labels = false;
+        let off = render(&c, &d, 640.0, 400.0, &ApproxMeasure);
+        c.data_labels = true;
+        let on = render(&c, &d, 640.0, 400.0, &ApproxMeasure);
+        let count = |p: &[Prim], v: &str| texts(p).iter().filter(|t| **t == v).count();
+        // One label per point: 3 series x 6 categories.
+        assert_eq!(texts(&on).len(), texts(&off).len() + 18, "{kind:?}");
+        for v in ["$12,000", "$38,000", "$9,000", "$27,000"] {
+            assert_eq!(count(&on, v), count(&off, v) + 1, "{kind:?}: {v}");
+        }
+        // Stacked areas label the source values, not cumulative totals (12,000 + 38,000).
+        assert_eq!(count(&on, "$50,000"), count(&off, "$50,000"), "{kind:?}: cumulative total shown");
+        let polys = |p: &[Prim]| p.iter().filter(|p| matches!(p, Prim::Polygon { .. })).cloned().collect::<Vec<_>>();
+        assert_eq!(polys(&on), polys(&off));
+        // The first series' labels are drawn after every area, so the next area can't cover them.
+        let last_poly = on.iter().rposition(|p| matches!(p, Prim::Polygon { .. }));
+        let first_label = on.iter().position(|p| matches!(p, Prim::Text { text, .. } if text == "$12,000"));
+        assert!(first_label > last_poly, "{kind:?}: label drawn under a later area");
+    }
+}