From 3301d59f442b1cd46b22337e48149b75a2787ed9 Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Mon, 12 Oct 2026 08:29:48 +0200
Subject: [PATCH] Fix histogram counts disagreeing with interval labels on bin
 boundaries (#546)

---
 crates/chart/src/render.rs |  8 +++++---
 crates/chart/src/tests.rs  | 22 ++++++++++++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/crates/chart/src/render.rs b/crates/chart/src/render.rs
index 63d30d982..0bb628ee1 100644
--- a/crates/chart/src/render.rs
+++ b/crates/chart/src/render.rs
@@ -1201,10 +1201,12 @@ fn histogram(ctx: &mut Ctx, chart: &Chart, data: &ChartData, area: Bx) {
         bins = 1;
     }
     let width = if bins == 1 && span.partial_cmp(&0.0) != Some(std::cmp::Ordering::Greater) { 1.0 } else { span / bins as f64 };
+    // Upper edge of each bin, exactly as shown in the labels below. Bins are right-inclusive (the first is closed on both sides), so a value
+    // equal to an edge belongs to the bin that ends there.
+    let edges: Vec<f64> = (0..bins).map(|b| if b + 1 == bins { hi.max(lo + b as f64 * width) } else { lo + (b + 1) as f64 * width }).collect();
     let mut counts = vec![0usize; bins];
     for v in &vals {
-        let b = if width > 0.0 { ((v - lo) / width).floor() } else { 0.0 };
-        let b = if b.is_finite() && b > 0.0 { (b as usize).min(bins - 1) } else { 0 };
+        let b = edges.partition_point(|e| e < v).min(bins - 1);
         if let Some(c) = counts.get_mut(b) {
             *c += 1;
         }
@@ -1213,7 +1215,7 @@ fn histogram(ctx: &mut Ctx, chart: &Chart, data: &ChartData, area: Bx) {
     let labels: Vec<String> = (0..bins)
         .map(|b| {
             let a = lo + b as f64 * width;
-            let e = if b + 1 == bins { hi.max(a) } else { a + width };
+            let e = edges.get(b).copied().unwrap_or(a);
             let open = if b == 0 { '[' } else { '(' };
             format!("{open}{}, {}]", f.format(a, Some(width / 10.0)), f.format(e, Some(width / 10.0)))
         })
diff --git a/crates/chart/src/tests.rs b/crates/chart/src/tests.rs
index a94267713..b38bf174d 100644
--- a/crates/chart/src/tests.rs
+++ b/crates/chart/src/tests.rs
@@ -877,6 +877,28 @@ fn treemap_and_histogram_shapes() {
     assert_eq!(bins, 10);
 }
 
+#[test]
+fn histogram_boundary_values_match_interval_labels() {
+    let heights = |vals: &[f64]| -> Vec<f32> {
+        let d = ChartData { categories: vec![], series: vec![series("h", vals, [10, 20, 30, 255], ChartKind::Histogram)] };
+        let prims = render(&chart(ChartKind::Histogram), &d, 600.0, 400.0, &ApproxMeasure);
+        prims.iter().filter_map(|p| if let Prim::Rect { h, fill: Some([10, 20, 30, 255]), .. } = p { Some(*h) } else { None }).collect()
+    };
+    // Labels are "[0, 7.5]" and "(7.5, 15.0]": 7.5 belongs to the first bar, so the counts are 2 and 1.
+    let h = heights(&[0.0, 7.5, 15.0]);
+    assert_eq!(h.len(), 2);
+    assert!((h[0] / h[1] - 2.0).abs() < 0.01, "{h:?}");
+    let h = heights(&[-15.0, -7.5, 0.0]);
+    assert!((h[0] / h[1] - 2.0).abs() < 0.01, "{h:?}");
+    // A value off the boundary still goes right: [0, 7.5] holds 1, (7.5, 15.0] holds 2.
+    let h = heights(&[0.0, 7.75, 15.0]);
+    assert!((h[1] / h[0] - 2.0).abs() < 0.01, "{h:?}");
+    // Five values, three bins [0,5] (5,10] (10,15]: counts 3, 1, 1.
+    let h = heights(&[0.0, 5.0, 5.0, 10.0, 15.0]);
+    assert_eq!(h.len(), 3);
+    assert!((h[0] / h[1] - 3.0).abs() < 0.01 && (h[1] - h[2]).abs() < 0.01, "{h:?}");
+}
+
 #[test]
 fn union_series_references() {
     let mut wb = Workbook::new();