implicit zero

docs/features/scales.md

@@ -702,7 +702,7 @@ A scale’s domain (the extent of its inputs, abstract values) and range (the ex
 * **reverse** - reverses the domain (or the range), say to flip the chart along *x* or *y*
 * **interval** - an interval or time interval (for interval data; see below)
 
-For most quantitative scales, the default domain is the [*min*, *max*] of all values associated with the scale. For the *radius* and *opacity* scales, the default domain is [0, *max*] to ensure a meaningful value encoding. For ordinal scales, the default domain is the set of all distinct values associated with the scale in natural ascending order; for a different order, set the domain explicitly or add a [**sort** option](#sort-mark-option) to an associated mark. For threshold scales, the default domain is [0] to separate negative and non-negative values. For quantile scales, the default domain is the set of all defined values associated with the scale. If a scale is reversed, it is equivalent to setting the domain as [*max*, *min*] instead of [*min*, *max*].
+For most quantitative scales, the default domain is the [*min*, *max*] of all values associated with the scale. For linear scales, the default domain is extended to include zero if within 7% of the domain extent; use **zero**: false to disable this, or **zero**: true to always include zero. For the *radius*, *opacity*, and *length* scales, the default domain is [0, *max*] to ensure a meaningful value encoding. For ordinal scales, the default domain is the set of all distinct values associated with the scale in natural ascending order; for a different order, set the domain explicitly or add a [**sort** option](#sort-mark-option) to an associated mark. For threshold scales, the default domain is [0] to separate negative and non-negative values. For quantile scales, the default domain is the set of all defined values associated with the scale. If a scale is reversed, it is equivalent to setting the domain as [*max*, *min*] instead of [*min*, *max*].
 
 The default range depends on the scale: for position scales (*x*, *y*, *fx*, and *fy*), the default range depends on the [plot’s size and margins](./plots.md). For color scales, there are default color schemes for quantitative, ordinal, and categorical data. For opacity, the default range is [0, 1]. And for radius, the default range is designed to produce dots of “reasonable” size assuming a *sqrt* scale type for accurate area representation: zero maps to zero, the first quartile maps to a radius of three pixels, and other values are extrapolated. This convention for radius ensures that if the scale’s data values are all equal, dots have the default constant radius of three pixels, while if the data varies, dots will tend to be larger.
 

src/scales/quantitative.js

@@ -58,7 +58,7 @@ export function createScaleQ(
     nice,
     clamp,
     zero,
-    domain = inferAutoDomain(key, channels),
+    domain = inferAutoDomain(key, channels, zero, type),
     unknown,
     round,
     scheme,
@@ -282,13 +282,24 @@ export function inferDomain(channels, f = finite) {
     : [0, 1];
 }
 
-function inferAutoDomain(key, channels) {
-  const type = registry.get(key);
-  return (type === radius || type === opacity || type === length ? inferZeroDomain : inferDomain)(channels);
-}
+function inferAutoDomain(key, channels, zero, type) {
+  const domain = inferDomain(channels, undefined);
+
+  if (zero === undefined && domain[0] * domain[1] > 0) {
+    // Default to zero for radius, opacity, and length scales
+    const scale = registry.get(key);
+    if (scale === radius || scale === opacity || scale === length) zero = true;
+    // If the zero option is not specified, then implicitly extend the domain to
+    // include zero if the minimum is less than 7% of the spread (and similarly
+    // for negative domains).
+    else if (type === "linear") {
+      const [min, max] = extent(domain);
+      zero = min > 0 ? min < 0.07 * (max - min) : max < 0 ? max > 0.07 * (min - max) : false;
+    }
+    if (zero) return [Math.min(0, domain[0]), Math.max(0, domain[1])];
+  }
 
-function inferZeroDomain(channels) {
-  return [0, channels.length ? max(channels, ({value}) => (value === undefined ? value : max(value, finite))) : 1];
+  return domain;
 }
 
 // We don’t want the upper bound of the radial domain to be zero, as this would