Add dataset indexes to tooltipModel

Feature Proposal

I think it would make sense to include an array of dataset indexes in the tooltipModel that is passed to any custom tooltip callback. The array should contain the index for each dataset that was included when calculating the position for the tooltip. Another way to phrase it is to say that the array should contain the index for each dataset that the tooltip belongs to.

Feature Use Case

Knowing which datasets the tooltip belongs to would make it possible to know what data to show in it if one e.g. uses the ‘nearest’ position option. For example, I have a custom tooltip (position: 'nearest') in which I want to show the current value it’s pointing to. In the tooltipModel, I just get a list of all the datasets that are relevant at the current index. By knowing which dataset the tooltip belongs to, I can easily pick out the right data from dataPoints. Currently, this is only possible by performing a (possibly faulty) comparison of caretY and dataPoints[...].y.

Possible Implementation

It looks like perhaps the positioners functions can easily be modified to return an array on top of the x and y coordinate, and then that array could easily be added to the model as something like “datasets”, “datasetIndexes”, or maybe “belongsTo”. If the change is this “simple”, I could try submitting a PR for it.

[godenji]

As of chart.js 3.0 you can get a single element array in a multi-dataset by setting tooltip mode to point.

tooltip: {
  enabled: true,
  mode: 'point',
  callbacks: {
    title: (xs: TooltipItem[]) => {
      // without mode: 'point' xs could consist of 1 or more elements
      return xs[0].dataset.label
    }
  }
}

[kurkle]

Closing as resolved