tip preferredAnchor

Author: mbostock

docs/marks/tip.md

@@ -182,7 +182,7 @@ Plot.plot({
 ```
 :::
 
-If you don’t specify an **anchor**, the tip mark will choose one automatically. It will prefer *top-left* if the tip fits; otherwise it will switch sides to try to contain the tip within the plot’s frame. When dynamically rendering the tip mark, say with the [pointer interaction](../interactions/pointer.md), the tip will also attempt to use the anchor it chose previously, making the tip more stable as you move the pointer and improving readability. In some cases, it may not be possible to fit the tip within the plot’s frame; consider setting the plot’s **style** to `overflow: visible;` to prevent the tip from being truncated.
+If you don’t specify an explicit **anchor**, the tip mark will choose one automatically, using the **preferredAnchor** <VersionBadge pr="1872" /> if it fits. The preferred anchor defaults to *bottom*, except when using the **tip** option and the [pointerY pointing mode](../interactions/pointer.md), in which case it defaults to *left*. In some cases, it may not be possible to fit the tip within the plot’s frame; consider setting the plot’s **style** to `overflow: visible;` to prevent the tip from being truncated.
 
 The tip mark is compatible with transforms that derive **x** and **y** dynamically from data, such as the [centroid transform](../transforms/centroid.md) which computes polygon centroids. Below, a map of the United States shows state names. We reduce the size of the tips by setting the **textPadding** option to 3 pixels instead of the default 8.
 

src/marks/tip.d.ts

@@ -62,6 +62,13 @@ export interface TipOptions extends MarkOptions, TextStyles {
    */
   anchor?: FrameAnchor;
 
+  /**
+   * If an explicit tip anchor is not specified, an anchor is chosen
+   * automatically such that the tip fits within the plot’s frame; if the
+   * preferred anchor fits, it is chosen.
+   */
+  preferredAnchor?: FrameAnchor | null;
+
   /**
    * How channel values are formatted for display. If a format is a string, it
    * is interpreted as a (UTC) time format for temporal channels, and otherwise

src/marks/tip.js

@@ -33,6 +33,7 @@ export class Tip extends Mark {
       y1,
       y2,
       anchor,
+      preferredAnchor = "bottom",
       monospace,
       fontFamily = monospace ? "ui-monospace, monospace" : undefined,
       fontSize,
@@ -65,7 +66,7 @@ export class Tip extends Mark {
       defaults
     );
     this.anchor = maybeAnchor(anchor, "anchor");
-    this.previousAnchor = this.anchor ?? "top-left";
+    this.preferredAnchor = maybeAnchor(preferredAnchor, "preferredAnchor");
     this.frameAnchor = maybeFrameAnchor(frameAnchor);
     this.textAnchor = impliedString(textAnchor, "middle");
     this.textPadding = +textPadding;
@@ -208,16 +209,26 @@ export class Tip extends Mark {
         (w = Math.round(w)), (h = Math.round(h)); // crisp edges
         let a = anchor; // use the specified anchor, if any
         if (a === undefined) {
-          a = mark.previousAnchor; // favor the previous anchor, if it fits
           const x = px(i) + ox;
           const y = py(i) + oy;
           const fitLeft = x + w + r * 2 < width;
           const fitRight = x - w - r * 2 > 0;
           const fitTop = y + h + m + r * 2 + 7 < height;
           const fitBottom = y - h - m - r * 2 > 0;
-          const ax = (/-left$/.test(a) ? fitLeft || !fitRight : fitLeft && !fitRight) ? "left" : "right";
-          const ay = (/^top-/.test(a) ? fitTop || !fitBottom : fitTop && !fitBottom) ? "top" : "bottom";
-          a = mark.previousAnchor = `${ay}-${ax}`;
+          a =
+            fitLeft && fitRight
+              ? fitTop && fitBottom
+                ? mark.preferredAnchor
+                : fitBottom
+                ? "bottom"
+                : "top"
+              : fitTop && fitBottom
+              ? fitLeft
+                ? "left"
+                : "right"
+              : (fitLeft || fitRight) && (fitTop || fitBottom)
+              ? `${fitBottom ? "bottom" : "top"}-${fitLeft ? "left" : "right"}`
+              : mark.preferredAnchor;
         }
         const path = this.firstChild; // note: assumes exactly two children!
         const text = this.lastChild; // note: assumes exactly two children!

src/plot.js

@@ -527,10 +527,11 @@ function inferTips(marks) {
     if (tipOptions) {
       if (tipOptions === true) tipOptions = {};
       else if (typeof tipOptions === "string") tipOptions = {pointer: tipOptions};
-      let {pointer: p} = tipOptions;
+      let {pointer: p, preferredAnchor: a} = tipOptions;
       p = /^x$/i.test(p) ? pointerX : /^y$/i.test(p) ? pointerY : pointer; // TODO validate?
       tipOptions = p(derive(mark, tipOptions));
       tipOptions.title = null; // prevent implicit title for primitive data
+      if (a === undefined) tipOptions.preferredAnchor = p === pointerY ? "left" : "bottom";
       const t = tip(mark.data, tipOptions);
       t.facet = mark.facet; // inherit facet settings
       t.facetAnchor = mark.facetAnchor; // inherit facet settings

test/output/tipLineX.svg

@@ -171,7 +171,12 @@ export async function tipHexbinExplicit() {
   });
 }
 
-export async function tipLine() {
+export async function tipLineX() {
+  const aapl = await d3.csv<any>("data/aapl.csv", d3.autoType);
+  return Plot.lineX(aapl, {y: "Date", x: "Close", tip: true}).plot();
+}
+
+export async function tipLineY() {
   const aapl = await d3.csv<any>("data/aapl.csv", d3.autoType);
   return Plot.lineY(aapl, {x: "Date", y: "Close", tip: true}).plot();
 }