address review: charge scatter Y and X values to the chart point budget as a pair

crates/chart/src/resolve.rs

@@ -246,7 +246,7 @@ pub fn resolve(wb: &Workbook, sheet: usize, chart: &Chart) -> ChartData {
     // One point budget for the whole chart, not one per series.
     let mut budget = MAX_CHART_POINTS;
     for (i, s) in chart.series.iter().take(MAX_SERIES).enumerate() {
-        let values = nums(wb, sheet, &s.values, &mut budget);
+        let mut values: Vec<Option<f64>> = eval_list_capped(wb, sheet, &s.values, budget).iter().map(to_num).collect();
         let name = s.name.as_deref().map(|n| series_name(wb, sheet, n)).filter(|n| !n.is_empty()).unwrap_or_else(|| format!("Series{}", i + 1));
         let color =
             s.color.as_ref().and_then(|c| c.resolve(&wb.theme)).map(|[r, g, b]| [r, g, b, 0xFF]).unwrap_or_else(|| series_color(&wb.theme, i));
@@ -255,16 +255,23 @@ pub fn resolve(wb: &Workbook, sheet: usize, chart: &Chart) -> ChartData {
             // X values with text in them (labels) are ignored: the points are numbered 1..n.
             (Some(f), true) => {
                 let xs = eval_list_capped(wb, sheet, f, budget);
-                // Only X values that are kept are charged to the budget.
                 if xs.iter().any(Value::is_text) {
                     None
                 } else {
-                    budget -= xs.len().min(budget);
-                    Some(xs.iter().map(to_num).collect())
+                    let mut xs: Vec<Option<f64>> = xs.iter().map(to_num).collect();
+                    // Y and X are kept as a pair: when both do not fit, each gets half, so a
+                    // series is never left with values and no X (which draws nothing).
+                    if values.len() + xs.len() > budget {
+                        values.truncate(budget / 2);
+                        xs.truncate(budget / 2);
+                    }
+                    budget -= xs.len();
+                    Some(xs)
                 }
             }
             _ => None,
         };
+        budget -= values.len();
         if categories.is_none()
             && let Some(f) = &s.categories
         {

crates/chart/src/tests.rs

@@ -1296,3 +1296,23 @@ fn rejected_text_x_values_are_not_charged_to_the_point_budget() {
     assert_eq!(d.series.len(), 40);
     assert!(d.series.iter().all(|s| s.values.len() == 100 && s.x.is_none()));
 }
+
+#[test]
+fn scatter_values_are_kept_with_their_x_values_when_the_budget_runs_out() {
+    // Y and X are charged as a pair: a series that gets values also gets X values, so it still draws.
+    let mut wb = Workbook::new();
+    let s = wb.sheet_mut(0).unwrap();
+    for col in 0..2 {
+        s.set_value(CellRef::new(0, col), Value::Number(1.0));
+        s.set_value(CellRef::new(59_999, col), Value::Number(2.0));
+    }
+    let mut c = chart(ChartKind::Scatter);
+    c.series = (0..40)
+        .map(|_| Series { values: "Sheet1!$A$1:$A$60000".into(), categories: Some("Sheet1!$B$1:$B$60000".into()), ..Default::default() })
+        .collect();
+    let d = resolve(&wb, 0, &c);
+    let total: usize = d.series.iter().map(|s| s.values.len() + s.x.as_ref().map_or(0, Vec::len)).sum();
+    assert!(total <= crate::resolve::MAX_CHART_POINTS);
+    assert!(d.series.iter().any(|s| s.values.len() < 60_000 && !s.values.is_empty()));
+    assert!(d.series.iter().all(|s| s.values.is_empty() || s.x.as_ref().is_some_and(|x| x.len() == s.values.len())));
+}