Auto mark: zero baseline default for rules

src/marks/auto.js

@@ -216,8 +216,10 @@ export function auto(data, options) {
 
   // If zero-ness is not specified, default based on whether the resolved mark
   // type will include a zero baseline. TODO Move this to autoSpec.
-  if (xZero === undefined) xZero = transform !== binX && (mark === barX || mark === areaX || mark === rectX);
-  if (yZero === undefined) yZero = transform !== binY && (mark === barY || mark === areaY || mark === rectY);
+  if (xZero === undefined)
+    xZero = X && transform !== binX && (mark === barX || mark === areaX || mark === rectX || mark === ruleY);
+  if (yZero === undefined)
+    yZero = Y && transform !== binY && (mark === barY || mark === areaY || mark === rectY || mark === ruleX);
 
   // In the case of filled marks (particularly bars and areas) the frame and
   // rules should come after the mark; in the case of stroked marks

test/plots/autoplot.ts

@@ -162,6 +162,11 @@ export async function autoRectColorReducer() {
   return Plot.auto(penguins, {x: "culmen_length_mm", color: {value: "island", reduce: "mode"}}).plot();
 }
 
+export async function autoRuleZero() {
+  const athletes = await d3.csv<any>("data/athletes.csv", d3.autoType);
+  return Plot.auto(athletes, {x: "date_of_birth", y: {value: "height", reduce: "mean"}, mark: "rule"}).plot();
+}
+
 export async function autoLineColor() {
   const aapl = await d3.csv<any>("data/aapl.csv", d3.autoType);
   return Plot.auto(aapl, {x: "Date", y: "Close", color: "Close"}).plot();