Easy way to add arbitrary text (e.g. units) to tooltip or text marks

[stenzengel]

Could you please implement an easy way to add arbitrary text (e.g. units) to tooltips or text marks, because the existing possibilities (proposed in #5122 (comment)) have several disadvantages:

• Derive a new field (in addition to the value) for the display text using the calculate transform and use that field in the tooltip or text mark
  Disadvantage: Duplication
• Custom format type (see https://vega.github.io/vega-lite/docs/config.html#custom-format-type)
  Disadvantages: Complex usage (needs JavaScript code) and no support in e.g. vl2svg (see Support expression for formatting axis #5122 (comment)) and VS Code

Possible enhancement alternatives come to my mind:

• New built-in formatType numberAndText:
  With this formatType the d3-number format is specified in braces and all other text is just copied:
  Usage:

"tooltip": [{
    "field": "Mean of energy",
    "format": "{.2f} kWh",
    "formatType": "numberAndText"
}]

Implementation:

vega.expressionFunction("numberAndText", function (datum, params) {
	return params.replace(/{(.*?)}/g, (_, p1) => d3.formatLocale(numberLocale).format(p1)(datum));
});

• Extension of the existing formatType number to use the same implementation as in the previous alternative, when the format specification contains braces:
  Since { and } do not occur as tokens in the d3-number specification (see https://github.com/d3/d3-format), this should not cause problems with d3-number specifications and has the advantage that "formatType": "number" can be emitted, if it is inferred by vega-lite (which should normally be the case).
  Usage:

"tooltip": [{
    "field": "Mean of energy",
    "format": "{.2f} kWh",
}]

• Implement formatExpr for tooltips and text marks/encodings.
  usage see https://vega.github.io/vega-lite/docs/axis.html#example-using-axis-labelexpr-to-display-initial-letters-of-month-name

[jeanfabrice]

Being able to add a custom text to express units (like Watt, Watt hour, Meter, Ampere, Liter, etc) to text marks, tooltips and axis labels would be a great addition!

I +1 this enhancement request. Thank you for raising this