From 88df1670ba88c2a7bc49944ed9ea5f3a18459014 Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sun, 11 Oct 2026 23:36:29 +0200
Subject: [PATCH] Scatter charts of two unlabelled columns plot the first as X
 for one series (#428)

---
 crates/engine/src/cmd/extra.rs  |  1 +
 crates/engine/src/cmd/insert.rs | 15 ++++++++++++++-
 crates/engine/src/tests.rs      | 25 +++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/crates/engine/src/cmd/extra.rs b/crates/engine/src/cmd/extra.rs
index 9f2565ac4..9249763f6 100644
--- a/crates/engine/src/cmd/extra.rs
+++ b/crates/engine/src/cmd/extra.rs
@@ -421,6 +421,7 @@ fn select_data(s: &mut Session, p: &Json) -> Result<Json> {
     with_chart(s, p, move |c| {
         if let Some((series, source, by_rows)) = rebuilt {
             c.series = series;
+            super::insert::scatter_x_from_first(c.kind, &mut c.series);
             c.source = Some(source);
             c.by_rows = by_rows;
         }
diff --git a/crates/engine/src/cmd/insert.rs b/crates/engine/src/cmd/insert.rs
index 16859c3dc..4e044efcb 100644
--- a/crates/engine/src/cmd/insert.rs
+++ b/crates/engine/src/cmd/insert.rs
@@ -573,6 +573,17 @@ pub fn series_from_range(sh: &Sheet, r: RangeRef, by_rows: bool) -> Vec<Series>
     out
 }
 
+/// A scatter plot of a block without category labels takes its first column (or row) as the X
+/// values of the others, as Excel does: two plain columns make one series of points, not two (#428).
+pub fn scatter_x_from_first(kind: ChartKind, series: &mut Vec<Series>) {
+    if matches!(kind, ChartKind::Scatter | ChartKind::ScatterLines) && series.len() > 1 && series[0].categories.is_none() {
+        let x = series.remove(0).values;
+        for se in series.iter_mut() {
+            se.categories = Some(x.clone());
+        }
+    }
+}
+
 /// Most cells a chart's source block may span (bounds the series it builds).
 const MAX_CHART_CELLS: u64 = 1_000_000;
 
@@ -661,10 +672,11 @@ pub fn build_chart(s: &Session, p: &Json) -> Result<(usize, Chart)> {
     let data_w = r.width();
     let by_rows = data_w > data_h + 1;
     let kind = chart_kind(&t, &sub);
-    let series = series_from_range(sh, r, by_rows);
+    let mut series = series_from_range(sh, r, by_rows);
     if series.is_empty() {
         return Err(EngineError::Other("Select data for the chart first.".into()));
     }
+    scatter_x_from_first(kind, &mut series);
     let id = d.wb.next_object_id();
     let at = cell_param_on(s, p, "at", sheet)?.unwrap_or_else(|| CellRef::new(r.start.row, r.end.col + 2));
     let title = str_param(p, "title").map(str::to_string).or_else(|| {
@@ -801,6 +813,7 @@ fn chart_switch(s: &mut Session, p: &Json) -> Result<Json> {
             c.source.get_or_insert(src);
             c.by_rows = by_rows;
             c.series = series;
+            scatter_x_from_first(c.kind, &mut c.series);
         }
         Ok(Json::Null)
     })
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index c9f4d0aa3..f5c0b7268 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -1554,6 +1554,31 @@ fn every_chart_kind_survives_xlsx() {
     }
 }
 
+#[test]
+fn scatter_of_two_columns_is_one_series_with_the_first_as_x() {
+    let mut s = s();
+    s.execute("range.setValues", json!({"range": "A1", "values": [["X", "Y"], [1, 10], [2, 20], [3, 30]]})).unwrap();
+    s.execute("insert.chart", json!({"type": "scatter", "range": "A1:B4"})).unwrap();
+    let c = s.doc().unwrap().wb.active().unwrap().charts[0].clone();
+    assert_eq!(c.series.len(), 1);
+    assert_eq!(c.series[0].values, "Sheet1!$B$2:$B$4");
+    assert_eq!(c.series[0].categories.as_deref(), Some("Sheet1!$A$2:$A$4"));
+    // Select Data on the scatter chart and Switch Row/Column rebuild the series the same way.
+    s.execute("chart.selectData", json!({"range": "A2:B4"})).unwrap();
+    let c = s.doc().unwrap().wb.active().unwrap().charts[0].clone();
+    assert_eq!(c.series.len(), 1);
+    assert_eq!(c.series[0].categories.as_deref(), Some("Sheet1!$A$2:$A$4"));
+    s.execute("range.setValues", json!({"range": "D1", "values": [[1, 2, 3, 4], [10, 20, 30, 40]]})).unwrap();
+    s.execute("chart.selectData", json!({"range": "D1:G2"})).unwrap();
+    s.execute("chart.switchRowColumn", json!({})).unwrap();
+    s.execute("chart.switchRowColumn", json!({})).unwrap();
+    let c = s.doc().unwrap().wb.active().unwrap().charts[0].clone();
+    assert!(c.by_rows);
+    assert_eq!(c.series.len(), 1);
+    assert_eq!(c.series[0].values, "Sheet1!$D$2:$G$2");
+    assert_eq!(c.series[0].categories.as_deref(), Some("Sheet1!$D$1:$G$1"));
+}
+
 #[test]
 fn whole_sheet_selection_stays_within_the_data() {
     // On an empty sheet with everything selected these filled or scanned every cell of the