Graphs: centered labels with an empty middle line stay centered

A centered label such as Q||1 has no baseline for its empty middle
line, so a label that keeps more leading was not moved back to its
place. The graph now centers a label whose first and last lines hold
characters, whatever the lines between them hold.

crates/engine/src/cmd/graph/labels.rs

@@ -112,10 +112,12 @@ impl Gen<'_> {
         let start = if middle { Point::new(at.x, at.y - half) } else { at };
         let mut t = self.text_object(role, start, &s.replace('|', "\n"), justify);
         // Kept leading or paragraph spacing moves the lines: put the middle of the first and last baselines on `at`. An
-        // empty first or last line has no baseline, and such a label stays where the default leading puts it.
+        // empty line has no baseline: an empty first or last line leaves the label where the default leading puts it.
         if middle && kept_para(self.styles, role).is_some() {
             let ys: Vec<f64> = t.baselines().map(|(a, _)| a.y).collect();
-            if let (Some(first), Some(last), true) = (ys.first(), ys.last(), ys.len() == lines) {
+            let ends_set = s.split('|').next().is_some_and(|l| !l.is_empty()) && s.split('|').next_back().is_some_and(|l| !l.is_empty());
+            let set = s.split('|').filter(|l| !l.is_empty()).count();
+            if let (Some(first), Some(last), true) = (ys.first(), ys.last(), ends_set && ys.len() == set) {
                 t.xf = Affine::translate((0.0, half - (first + last) / 2.0)) * t.xf;
             }
         }

crates/engine/src/cmd/graph/tests.rs

@@ -4370,6 +4370,33 @@ fn a_centered_label_with_kept_leading_stays_centered_when_the_graph_is_drawn_aga
     assert!((after - before).abs() < 0.01, "the two-line category label keeps its middle: {before} {after}");
 }
 
+#[test]
+fn a_centered_label_with_an_empty_middle_line_and_kept_leading_stays_centered() {
+    let mut s = Session::new();
+    s.execute("file.new", &json!({"width": 800, "height": 800})).unwrap();
+    let csv = ",2024,2025\nQ||1,3,2\nQ2,5,4\nQ3,-1,6";
+    let id = NodeId(
+        s.execute("graph.create", &json!({"type": "bar", "x": 100, "y": 100, "width": 300, "height": 200, "csv": csv})).unwrap()["id"]
+            .as_u64()
+            .unwrap(),
+    );
+    let middle = |s: &Session| {
+        let n = s.doc().unwrap().doc.node(id).unwrap().clone();
+        let label =
+            group(&n, "Axes").children().unwrap().iter().find(|c| matches!(&c.kind, NodeKind::Text(t) if t.plain_text() == "Q\n\n1")).unwrap();
+        let b = label.geometric_bounds().unwrap();
+        (b.y0 + b.y1) / 2.0
+    };
+    let before = middle(&s);
+    let l = text_id(&s, id, "Axes", "Q\n\n1");
+    s.execute("select.set", &json!({"ids": [l.0]})).unwrap();
+    s.execute("text.setFormat", &json!({"autoLeading": 400})).unwrap();
+    s.execute("select.set", &json!({"ids": [id.0]})).unwrap();
+    s.execute("graph.setData", &json!({"id": id.0, "csv": csv})).unwrap();
+    let after = middle(&s);
+    assert!((after - before).abs() < 0.01, "the three-line category label keeps its middle: {before} {after}");
+}
+
 #[test]
 fn a_legend_across_top_measures_its_labels_with_system_layout() {
     let mut s = Session::new();