T14c, T15 review fixes: Tab steps past merges, empty charts lay out nothing, flat domains widen by size, pointed parts let go of shrunk data, signed waterfall totals, sunburst children take their share, today stays on the timeline

src/charts/stats.rs

@@ -125,14 +125,14 @@ pub(crate) enum Step {
     Total,
 }
 
-/// Each change floating from the running total, bottom and top; a total stands on zero at the sum so far.
+/// Each change floating from the running total, bottom and top; a total stands on zero, reaching up or down to the sum so far.
 pub(crate) fn steps(changes: &[(f64, bool)]) -> Vec<(f64, f64, Step)> {
-    let mut running = 0.0;
+    let mut running = 0.0f64;
     changes
         .iter()
         .map(|(value, total)| {
             if *total {
-                return (0.0, running, Step::Total);
+                return (running.min(0.0), running.max(0.0), Step::Total);
             }
             let from = running;
             running += value;
@@ -184,6 +184,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_total_below_zero_hangs_down_from_it() {
+        let bars = steps(&[(-10.0, false), (0.0, true)]);
+        let (bottom, top, step) = bars[1];
+        assert_eq!(
+            (bottom, top, step),
+            (-10.0, 0.0, Step::Total),
+            "the bar runs from the total up to zero"
+        );
+    }
+
     #[test]
     fn a_waterfall_floats_changes_and_stands_totals_on_zero() {
         let bars = steps(&[(100.0, false), (-30.0, false), (0.0, true), (20.0, false)]);