Simple tips for simple data

Context: #1657 showed a bug due to the way we handle primitive data in the tip mark. #1658 fixes it.

I'd like to discuss an alternative, which is not based on the data being an iterable of primitives, but on the following heuristic: if there is only 1 line of tip, we ignore the channel name in the tip.

For instance, it would avoid showing "date of birth" in this tip, which is obviously redundant:

For the new test case added in #1658, this heuristic would show "x: a\nfrequency: 1", because the y channel is “meaningful” (admittedly it's always 1 because all the letters are different, but it would be meaningful if we passed interesting data).

And for cases where an inferred channel is deemed "uninteresting" (like x for a lineY([primitive values])), we might want a hint that tells the tip mark to ignore that channel.

typical examples:

• Plot.lineY([1, 2, 3, 4, 1, 0, 2], { tip: true }).plot(); // we want 1, 2, 3… in the tip
• Plot.cellX([1, 2, 3, 4, 1, 0, 2], { tip: true }).plot(); // we want 1, 2, 3… in the tip, with a color swatch

I've played a bit with the idea, and could only come up with this very ugly code that even ChatGPT would be ashamed of:

--- a/src/marks/tip.js
+++ b/src/marks/tip.js
@@ -24,7 +24,6 @@ const ignoreChannels = new Set(["geometry", "href", "src", "ariaLabel"]);
 export class Tip extends Mark {
   constructor(data, options = {}) {
     if (options.tip) options = {...options, tip: false};
-    if (options.title === undefined && isIterable(data) && isTextual(data)) options = {...options, title: identity};
     const {
       x,
       y,
@@ -171,12 +170,15 @@ export class Tip extends Mark {
               this.setAttribute("stroke", "none");
               // iteratively render each channel value
               const names = new Set();
+              let lines = 0;
               for (const line of format(sources, i)) {
                 const name = line.name;
                 if (name && names.has(name)) continue;
                 else names.add(name);
                 renderLine(that, line);
+                lines++;
               }
+              if (lines === 1) that.html(""), renderLine(that, {...[...format(sources, i)][0], name: ""});
             })
           )
       );

Start with src/marks/tip.js and the primitive-data cases described here, then review #1657 and #1658 for the current behavior. Resolve which tip heuristic and inferred-channel handling is intended, add coverage for the lineY and cellX examples, and verify the rendered tip text and color swatch match the accepted behavior.