same logic for the vertical case + tests

Author: Fil

src/marks/tip.js

@@ -240,6 +240,10 @@ export class Tip extends Mark {
               ? "left"
               : fitRight
               ? "right"
+              : fitTop
+              ? "top"
+              : fitBottom
+              ? "bottom"
               : mark.preferredAnchor;
         }
         const path = this.firstChild; // note: assumes exactly two children!

test/output/tipAnchorOverflowY.svg

@@ -318,7 +318,7 @@ import "./text-overflow.js";
 import "./this-is-just-to-say.js";
 import "./tick-format.js";
 import "./time-axis.js";
-import "./tip-anchor-overflow.js";
+import "./tip-anchor.js";
 import "./tip-format.js";
 import "./tip.js";
 import "./title.js";

test/output/tipAnchorPositions.svg

@@ -0,0 +1,75 @@
+import * as Plot from "@observablehq/plot";
+import {test} from "test/plot";
+
+// Test tip anchor selection near the right edge of the chart.
+test(async function tipAnchorOverflow() {
+  const plot = Plot.rectX([1, 1, 1, 1, 1], {
+    x: Plot.identity,
+    fill: Plot.indexOf,
+    title: () =>
+      "Lorem ipsum lorem ipsum lorem ipsum Lorem ipsum lorem ipsum lorem ipsum Lorem ipsum lorem ipsum lorem ipsum",
+    tip: true
+  }).plot({height: 80, marginTop: 20, axis: null});
+  plot.dispatchEvent(
+    new PointerEvent("pointermove", {
+      pointerType: "mouse",
+      clientX: 580,
+      clientY: 30
+    })
+  );
+  return Object.assign(plot, {ready: new Promise((resolve) => setTimeout(resolve, 100))});
+});
+
+// Test tip anchor selection near the bottom edge of the chart.
+test(async function tipAnchorOverflowY() {
+  const plot = Plot.rectY([1, 1, 1, 1, 1], {
+    y: Plot.identity,
+    fill: Plot.indexOf,
+    title: () =>
+      "Lorem ipsum lorem ipsum lorem ipsum Lorem ipsum lorem ipsum lorem ipsum Lorem ipsum lorem ipsum lorem ipsum",
+    tip: {lineWidth: 5}
+  }).plot({width: 80, marginLeft: 20, axis: null});
+  plot.dispatchEvent(
+    new PointerEvent("pointermove", {
+      pointerType: "mouse",
+      clientX: 30,
+      clientY: 20
+    })
+  );
+  return Object.assign(plot, {ready: new Promise((resolve) => setTimeout(resolve, 100))});
+});
+
+// Test tip anchor selection at all 9 positions across the chart.
+test(async function tipAnchorPositions() {
+  const data = [];
+  for (const px of [40, 320, 600]) {
+    for (const py of [20, 100, 180]) {
+      data.push({px, py});
+    }
+  }
+  const plot = Plot.plot({
+    width: 640,
+    height: 200,
+    axis: null,
+    inset: 20,
+    marks: [
+      Plot.dot(data, {x: "px", y: "py", r: 5, fill: "currentColor"}),
+      Plot.tip(data, {
+        x: "px",
+        y: "py",
+        title: () => "A tip that is wide enough to test anchor fitting behavior across positions"
+      })
+    ]
+  });
+  // Dispatch a pointer event to trigger all tips
+  for (const {px, py} of data) {
+    plot.dispatchEvent(
+      new PointerEvent("pointermove", {
+        pointerType: "mouse",
+        clientX: px,
+        clientY: py
+      })
+    );
+  }
+  return Object.assign(plot, {ready: new Promise((resolve) => setTimeout(resolve, 100))});
+});