Create custom tooltip from element instead of HTML string

[Radiergummi]

Summary

Allow to provide an element instance from the custom tooltip callback in addition to just an HTML string. This would make it a lot easier to integrate with other libraries and frontend stacks.

API Changes

Really just this:

-   tooltipEl.innerHTML = fn({
+   const customTooltip = fn({
      ctx: this.ctx,
      series: w.globals.series,
      seriesIndex: i,
      dataPointIndex: j,
      y1,
      y2,
      w,
    })
+
+   if (typeof customTooltip === 'string') {
+     tooltipEl.innerHTML = customTooltip
+   } else {
+     tooltipEl.appendChild(customTooltip)
+   }

src/modules/tooltip/Labels.js, Lines 527–535

Intended Use Case

This allows the custom tooltip callback to not only return an HTML string, but also an HTML element instance, which solves a slew of issues and user questions, and allows more advanced integration, such as using a prop in React, or a slot in a Vue component, to render a tooltip.

The API change doesn't introduce any backwards incompatibilities.

[junedchhipa]

Great suggestion, will implement this.