Charts: draw, read and write value-axis minor gridlines

crates/model/src/chart.rs

@@ -159,6 +159,8 @@ pub struct Chart {
     pub title_size: Option<f64>,
     pub data_labels: bool,
     pub gridlines: bool,
+    /// `c:minorGridlines` on the value axis: five to a major unit, between the major ones.
+    pub minor_gridlines: bool,
     pub axis_titles: (Option<String>, Option<String>),
     /// Chart style number (1–48) and colour palette (`colorful1`, `monochrome3`…).
     pub style: u32,
@@ -204,6 +206,7 @@ impl Default for Chart {
             title_size: None,
             data_labels: false,
             gridlines: true,
+            minor_gridlines: false,
             axis_titles: (None, None),
             style: 2,
             palette: "colorful1".into(),

crates/pptx/src/read/chart.rs

@@ -266,6 +266,7 @@ pub(crate) fn chart_from(root: &El) -> Option<Chart> {
     let cat_ax = pa.child("catAx").or_else(|| pa.child("dateAx")).or_else(|| pa.children_named("valAx").find(|a| !vertical(a)));
     if let Some(v) = val_ax {
         c.gridlines = v.child("majorGridlines").is_some();
+        c.minor_gridlines = v.child("minorGridlines").is_some();
         let sc = v.child("scaling");
         c.y_min = sc.and_then(|s| s.child_val("min")).and_then(|v| v.parse().ok()).filter(|v: &f64| v.is_finite());
         c.y_max = sc.and_then(|s| s.child_val("max")).and_then(|v| v.parse().ok()).filter(|v: &f64| v.is_finite());
@@ -310,6 +311,16 @@ fn text_size(e: Option<&El>) -> Option<f64> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn minor_gridlines_are_read_and_written_back() {
+        let mut c = Chart::sample(ChartType::Line);
+        c.minor_gridlines = true;
+        let back = read_chart_xml(&crate::write::chart::chart_xml(&c)).unwrap();
+        assert!(back.gridlines && back.minor_gridlines);
+        c.minor_gridlines = false;
+        assert!(!read_chart_xml(&crate::write::chart::chart_xml(&c)).unwrap().minor_gridlines);
+    }
+
     #[test]
     fn legend_position_and_overlay_are_read() {
         let legend = r#"<c:legend><c:legendPos val="r"/><c:overlay val="1"/></c:legend>"#;

crates/pptx/src/write/chart.rs

@@ -286,6 +286,9 @@ fn axes(w: &mut W, c: &Chart, g: Group) {
     if c.gridlines {
         w.empty0("c:majorGridlines");
     }
+    if c.minor_gridlines {
+        w.empty0("c:minorGridlines");
+    }
     if let Some(t) = &c.axis_titles.1 {
         title(w, t);
     }

crates/pptx/src/write/mod.rs

@@ -1,6 +1,6 @@
 //! PPTX export: [`Presentation`] → package.
 
-mod chart;
+pub(crate) mod chart;
 pub(crate) mod dml;
 mod shapes;
 mod theme;

crates/render/src/chart.rs

@@ -282,6 +282,26 @@ pub fn draw(ctx: &mut RenderContext, rctx: &Ctx, c: &Chart, m: Affine, w: f64, h
         (true, Some((min2, max2, _))) => plot.y1 - (v - min2) / (max2 - min2).max(1e-9) * plot.height(),
         _ => to_v(v),
     };
+    // Minor gridlines, under the major ones: five to a major unit.
+    if c.minor_gridlines {
+        let minor = step / 5.0;
+        let n = (((max - min) / minor.max(1e-12)).round() as i64).clamp(1, 500);
+        let mut g = BezPath::new();
+        for k in (0..=n).filter(|k| k % 5 != 0) {
+            let pos = to_v(min + k as f64 * minor);
+            if horiz {
+                g.move_to((pos, plot.y0));
+                g.line_to((pos, plot.y1));
+            } else {
+                g.move_to((plot.x0, pos));
+                g.line_to((plot.x1, pos));
+            }
+        }
+        ctx.set_transform(m);
+        ctx.set_paint(color(Rgba::rgb(0xF2, 0xF2, 0xF2), 1.0));
+        ctx.set_stroke(kurbo::Stroke::new(0.75));
+        ctx.stroke_path(&g);
+    }
     // Gridlines and value labels.
     for v in ticks((min, max, step)) {
         let pos = to_v(v);