Charts: draw the axes' titles

crates/render/src/chart.rs

@@ -246,6 +246,23 @@ pub fn draw(ctx: &mut RenderContext, rctx: &Ctx, c: &Chart, m: Affine, w: f64, h
     // Bars are drawn first and lines over them; a combo chart has both.
     let bar_kind = c.series.iter().map(|s| c.series_kind(s)).find(|k| k.is_bar_like());
     let horiz = if combo { bar_kind.is_some_and(ChartType::horizontal) } else { c.kind.horizontal() };
+    // Axis titles: the left one turned to read upwards, the bottom one centred under the axis.
+    // On a horizontal bar chart the category axis is the left one.
+    let (left_title, bottom_title) =
+        if horiz { (c.axis_titles.0.as_deref(), c.axis_titles.1.as_deref()) } else { (c.axis_titles.1.as_deref(), c.axis_titles.0.as_deref()) };
+    let band_h = base * 1.6;
+    if let Some(t) = bottom_title.filter(|t| !t.is_empty()) {
+        let r = Rect::new(area.x0, area.y1 - band_h, area.x1, area.y1);
+        label(ctx, rctx, m, t, r, base, Align::Center, Anchor::Bottom, text_col, true);
+        area.y1 -= band_h + base * 0.3;
+    }
+    if let Some(t) = left_title.filter(|t| !t.is_empty()) {
+        let (cx, cy) = (area.x0 + band_h / 2.0, (area.y0 + area.y1) / 2.0);
+        let turned = m * Affine::translate((cx, cy)) * Affine::rotate(-std::f64::consts::FRAC_PI_2);
+        let half = area.height() / 2.0;
+        label(ctx, rctx, turned, t, Rect::new(-half, -band_h / 2.0, half, band_h / 2.0), base, Align::Center, Anchor::Middle, text_col, true);
+        area.x0 += band_h + base * 0.3;
+    }
     // As many major units as the value axis has room for labels (PowerPoint's automatic unit).
     let axis_len = if horiz { area.width() - base * 4.5 } else { area.height() - base * 1.8 };
     let max_intervals = (axis_len / if horiz { base * 6.0 } else { base * 1.9 }).floor().clamp(2.0, 12.0) as usize;

crates/render/src/tests.rs

@@ -155,6 +155,28 @@ fn combo_chart_draws_columns_and_a_line_on_its_own_axis() {
     assert!(col[0] < 250 || col[1] < 250 || col[2] < 250, "no column: {col:?}");
 }
 
+#[test]
+fn chart_axis_titles_are_drawn() {
+    let draw = |titles: (Option<String>, Option<String>)| {
+        let mut c = Chart::sample(ChartType::Column);
+        c.title = None;
+        c.legend = None;
+        c.axis_titles = titles;
+        let sh = Shape { id: ShapeId(624), xfrm: Some(Xfrm::new(0.0, 0.0, 960.0, 540.0)), kind: ShapeKind::Chart(Box::new(c)), ..Default::default() };
+        render_slide(&deck_with(vec![sh]), 0, &RenderOpts { scale: 1.0, ..Default::default() })
+    };
+    let ink = |img: &Image, r: (u32, u32, u32, u32)| {
+        (r.0..r.2).flat_map(|x| (r.1..r.3).map(move |y| (x, y))).filter(|(x, y)| img.pixel(*x, *y)[0] < 160).count()
+    };
+    let bare = draw((None, None));
+    let titled = draw((Some("Month".into()), Some("Revenue".into())));
+    // A turned "Revenue" down the left edge, "Month" along the bottom.
+    let left = (38, 150, 62, 390);
+    let bottom = (380, 490, 580, 520);
+    assert!(ink(&titled, left) > ink(&bare, left) + 30, "value axis title");
+    assert!(ink(&titled, bottom) > ink(&bare, bottom) + 30, "category axis title");
+}
+
 #[test]
 fn placeholder_prompts_only_in_edit_view() {
     let p = Presentation::default();