no implicit title on derived tip (#1658)

Author: mbostock

src/plot.js

@@ -492,7 +492,9 @@ function inferTips(marks) {
     const t = mark.tip;
     if (t) {
       const p = t === "x" ? pointerX : t === "y" ? pointerY : pointer;
-      tips.push(tip(mark.data, p(derive(mark)))); // TODO tip options?
+      const options = p(derive(mark)); // TODO tip options?
+      options.title = null; // prevent implicit title for primitive data
+      tips.push(tip(mark.data, options));
     }
   }
   return tips;

test/output/tipGroupPrimitives.svg

@@ -122,6 +122,14 @@ export async function tipGeoCentroid() {
   });
 }
 
+export async function tipGroupPrimitives() {
+  return Plot.plot({
+    height: 80,
+    x: {type: "band"},
+    marks: [Plot.barY("de156a2fc8", Plot.groupX({y: "count"}, {x: (d) => d, tip: true}))]
+  });
+}
+
 export async function tipHexbin() {
   const olympians = await d3.csv<any>("data/athletes.csv", d3.autoType);
   return Plot.hexagon(olympians, Plot.hexbin({r: "count"}, {x: "weight", y: "height", tip: true})).plot();