Widen the Y-axis gap threshold to 2% of the span

A nudge only large enough to break an exact tie left the gap unchanged
whenever the data sat just above a tick rather than on it, so a chart could
still render with a hairline of headroom.

Because ECharts rounds out to a whole tick afterwards, this constant does not
set the size of the resulting gap — it decides which axes count as too flat to
leave alone. At 2% the worst case over the tick cycle goes from nothing to
around 3% of the plot, while every measured chart keeps the framing it had.

Author: MindFreeze

src/components/chart/y-axis-fraction-digits.ts

@@ -9,14 +9,14 @@ const NEGLIGIBLE_RANGE_RATIO = 1e-10;
 // precision derived here cannot drift from the ticks it renders.
 const SPLIT_NUMBER = 5;
 
-// Fraction of the data span added at each end, so that ECharts' tick rounding —
-// which floors the minimum and ceils the maximum to a tick multiple — always has
-// something to round away. Quantized states otherwise land exactly on a tick,
-// the rounding changes nothing, and area-filled series collapse to zero height.
-// Any value between roughly 1e-15 and 1 / SPLIT_NUMBER works: large enough to
-// survive float64 addition at the data's magnitude, small enough that it can
-// never cross a tick by itself.
-const GAP_FRACTION_OF_SPAN = 1e-6;
+// How thin a gap between the data and the plot edge counts as no gap at all,
+// as a fraction of the data span. ECharts floors the axis minimum and ceils the
+// maximum to a tick multiple, which usually leaves headroom, but quantized
+// states often land exactly on a tick and get none — collapsing area-filled
+// series, which are drawn from their value down to the axis minimum. Widening
+// the extent by this much before that rounding bumps any axis with less
+// headroom out to a full tick, and leaves the rest where they are.
+const GAP_FRACTION_OF_SPAN = 0.02;
 
 // Derive the number of decimal digits to use for Y-axis labels from the
 // observed data range, by asking ECharts for the same tick interval it will

test/components/chart/y-axis-boundary-gap.test.ts

@@ -58,8 +58,10 @@ describe("Y-axis tick nudge", () => {
     expect(renderExtent([18, 21.2, 19], withGap())).toEqual([17.5, 21.5]);
   });
 
-  it("leaves an axis that already had a gap untouched", () => {
-    const data = [1.11109, 1.85849, 1.4];
+  it("leaves an axis with real headroom untouched", () => {
+    // Its minimum sits well clear of the tick below it, so the rounding it
+    // already gets is enough and the widened extent floors to the same place.
+    const data = [18.3, 21.2, 19.5];
     expect(renderExtent(data, withGap())).toEqual(
       renderExtent(data, { scale: true })
     );
@@ -71,7 +73,7 @@ describe("Y-axis tick nudge", () => {
     expect(
       renderExtent([0, 3500, 1200], {
         scale: true,
-        boundaryGap: [1e-6, 1e-6],
+        boundaryGap: [0.02, 0.02],
       })[0]
     ).toBe(-1000);
   });

test/components/chart/y-axis-fraction-digits.test.ts

@@ -142,7 +142,7 @@ describe("createYAxisPrecisionBounds", () => {
     const { min, max, boundaryGap, splitNumber } = makeBounds();
 
     // The gap is applied by ECharts, so the bounds stay auto-scaled.
-    expect(boundaryGap).toEqual([1e-6, 1e-6]);
+    expect(boundaryGap).toEqual([0.02, 0.02]);
     // Pinned rather than assumed, since the precision here is derived from it.
     expect(splitNumber).toBe(5);
     expect(min({ min: 18, max: 21.2 })).toBeUndefined();
@@ -158,7 +158,7 @@ describe("createYAxisPrecisionBounds", () => {
     expect(max({ min: 0, max: 3500 })).toBeUndefined();
 
     // A minimum clear of the gap is left to ECharts.
-    expect(min({ min: 0.01, max: 3500 })).toBeUndefined();
+    expect(min({ min: 100, max: 3500 })).toBeUndefined();
   });
 
   it("anchors an all-negative series at zero from above", () => {