First pass at #768, works for linear-integer

TODO
* should trigger on the interval option being integer
* band-integer (e.g., yearly-request)
* find a nicer way to pass the information up
* test for the [-10, 10] cases—maybe more what about 10-30?

Author: Fil

src/marks/axis.js

@@ -381,6 +381,7 @@ function axisTextKy(
       dx: anchor === "left" ? +dx - tickSize - tickPadding + +insetLeft : +dx + +tickSize + +tickPadding - insetRight
     },
     function (scale, ticks, channels) {
+      if (scale.type === "linear-integer" && tickFormat === undefined) tickFormat = "d";
       if (fontVariant === undefined) this.fontVariant = inferFontVariant(scale);
       if (text === undefined) channels.text = inferTextChannel(scale, ticks, tickFormat);
     }
@@ -428,6 +429,7 @@ function axisTextKx(
       dy: anchor === "bottom" ? +dy + +tickSize + +tickPadding - insetBottom : +dy - tickSize - tickPadding + +insetTop
     },
     function (scale, ticks, channels) {
+      if (scale.type === "linear-integer" && tickFormat === undefined) tickFormat = "d";
       if (fontVariant === undefined) this.fontVariant = inferFontVariant(scale);
       if (text === undefined) channels.text = inferTextChannel(scale, ticks, tickFormat);
     }
@@ -505,12 +507,17 @@ function axisMark(mark, k, ariaLabel, data, options, initialize) {
       if (!scale) throw new Error(`missing scale: ${k}`);
       let {ticks, tickSpacing, interval} = options;
       if (isTemporalScale(scale) && typeof ticks === "string") (interval = ticks), (ticks = undefined);
+
+      const tickFunction =
+        scale.type === "linear-integer"
+          ? (ticks) => scale.ticks(ticks).filter((d) => d === Math.floor(d))
+          : scale.ticks;
       if (data == null) {
         if (isIterable(ticks)) {
           data = arrayify(ticks);
         } else if (scale.ticks) {
           if (ticks !== undefined) {
-            data = scale.ticks(ticks);
+            data = tickFunction(ticks);
           } else {
             interval = maybeInterval(interval === undefined ? scale.interval : interval, scale.type);
             if (interval !== undefined) {
@@ -523,7 +530,7 @@ function axisMark(mark, k, ariaLabel, data, options, initialize) {
             } else {
               const [min, max] = extent(scale.range());
               ticks = (max - min) / (tickSpacing === undefined ? (k === "x" ? 80 : 35) : tickSpacing);
-              data = scale.ticks(ticks);
+              data = tickFunction(ticks);
             }
           }
         } else {

src/scales.js

@@ -60,7 +60,7 @@ export function Scales(
   const scales = {};
   for (const [key, channels] of channelsByScale) {
     const scaleOptions = options[key];
-    const scale = Scale(key, channels, {
+    const mutableOptions = {
       round: registry.get(key) === position ? round : undefined, // only for position
       nice,
       clamp,
@@ -69,7 +69,8 @@ export function Scales(
       padding,
       projection,
       ...scaleOptions
-    });
+    };
+    const scale = Scale(key, channels, mutableOptions);
     if (scale) {
       // populate generic scale options (percent, transform, insets)
       let {
@@ -300,6 +301,7 @@ function Scale(key, channels = [], options = {}) {
     case "cyclical":
     case "sequential":
     case "linear":
+    case "linear-integer":
     case "sqrt":
     case "threshold":
     case "quantile":
@@ -342,6 +344,7 @@ function Scale(key, channels = [], options = {}) {
     case "cyclical":
     case "sequential":
     case "linear":
+    case "linear-integer":
       return ScaleLinear(key, channels, options);
     case "sqrt":
       return ScaleSqrt(key, channels, options);
@@ -446,7 +449,24 @@ function inferScaleType(key, channels, {type, domain, range, scheme, pivot, proj
   if (values.some(isOrdinal)) return asOrdinalType(kind);
   if (values.some(isTemporal)) return "utc";
   if (kind === color && (pivot != null || isDivergingScheme(scheme))) return "diverging";
-  return "linear";
+
+  // If a large sample of values reveals integers all in [1500, 2200]—or all in
+  // [-10, 10]—, we prefer an "integer" formatter without commas nor decimal
+  // ticks.
+  let testYear = true;
+  let testSmallInt = true;
+  for (const value of values) {
+    let N = 40;
+    for (const v of value) {
+      if (v != null) {
+        if (v !== Math.floor(v)) return "linear";
+        testYear &= v < 1500 || v > 2200;
+        testSmallInt &= v < -10 || v > 10;
+        if (N-- < 0) continue;
+      }
+    }
+  }
+  return testYear || testSmallInt ? "linear-integer" : "linear";
 }
 
 // Positional scales default to a point scale instead of an ordinal scale.

test/plots/energy-production.js

@@ -32,7 +32,6 @@ export default async function () {
     .map((d) => ({...d, Year: +d.YYYYMM.slice(0, 4), Value: +d.Value}));
   return Plot.plot({
     x: {
-      tickFormat: "d",
       label: null
     },
     y: {