auto-round based on wasted space (#558)

* auto-round based on wasted error

* don’t round ordinal scales

Author: mbostock

src/scales.js

@@ -68,11 +68,25 @@ function autoScaleRangeY(scale, dimensions) {
 }
 
 function autoScaleRound(scale) {
-  if (scale.round === undefined && isOrdinalScale(scale) && scale.scale.step() >= 5) {
+  if (scale.round === undefined && isBandScale(scale) && roundError(scale) <= 30) {
     scale.scale.round(true);
   }
 }
 
+// If we were to turn on rounding for this band or point scale, how much wasted
+// space would it introduce (on both ends of the range)? This must match
+// d3.scaleBand’s rounding behavior:
+// https://github.com/d3/d3-scale/blob/83555bd759c7314420bd4240642beda5e258db9e/src/band.js#L20-L32
+function roundError({scale}) {
+  const n = scale.domain().length;
+  const [start, stop] = scale.range();
+  const paddingInner = scale.paddingInner ? scale.paddingInner() : 1;
+  const paddingOuter = scale.paddingOuter ? scale.paddingOuter() : scale.padding();
+  const m = n - paddingInner;
+  const step = Math.abs(stop - start) / Math.max(1, m + paddingOuter * 2);
+  return (step - Math.floor(step)) * m;
+}
+
 function piecewiseRange({scale, range}) {
   const length = scale.domain().length;
   if (!(length > 2)) return range;
@@ -179,6 +193,10 @@ export function isOrdinalScale({type}) {
   return type === "ordinal" || type === "point" || type === "band";
 }
 
+function isBandScale({type}) {
+  return type === "point" || type === "band";
+}
+
 export function isDivergingScale({type}) {
   return /^diverging($|-)/.test(type);
 }

test/plots/google-trends-ridgeline.js

@@ -14,6 +14,7 @@ export default async function() {
       axis: null
     },
     fy: {
+      round: true,
       label: null
     },
     facet: {

test/plots/metro-unemployment-ridgeline.js

@@ -11,6 +11,7 @@ export default async function() {
       axis: null
     },
     fy: {
+      round: true,
       label: null
     },
     facet: {