[BUG?] Too many datapoints being hovered/in tooltip (mode: 'x')

[mwoco]

If you create a chart with hover-mode 'x'/tooltip-mode 'x' and many values in its datasets (try 500), tooltips will be looking like this:

As you can see in the image more than one datapoint per dataset is being 'hovered' and congruously the tooltip shows all of them.

I would expect to see only the one datapoint per dataset, which is closest to the mouse cursor, (preferred method) OR the API to give me a convenient way to limit the number of datapoints per dataset.

Situation can be improved by reducing the hoverRadius. But this also decreases usability. Is the 'filter' callback perhaps an option here? The 'index' mode is not an option in this scenario, because values can be missing from time to time in one dataset or the other.

Thank you for your efforts!

[etimberg]

@mwoco do you have a fiddle that reproduces this?

There is a filter on the tooltip that could be used.

[mwoco]

https://codepen.io/anon/pen/rJePWV?editors=1111

[MPierre9]

Anyone working on this? Do you mind if I try to fix it?

[etimberg]

@MPierre9 you're welcome to have a look at it. I would look at the code in https://github.com/chartjs/Chart.js/blob/master/src/core/core.interaction.js

[MPierre9]

Thanks for the tip @etimberg, it really helped!

[instantaphex]

I realize that there is a PR in the pipeline currently that will allow for the tooltip filter to handle this a bit better, but I needed a solution ASAP and I figured I'd share what I did in case anyone is looking for something quick and dirty until #7324 gets merged in.

WARNING: I'm accessing private variables of the chart, so this really only should be used as a temporary measure until the PR is merged in. If this blows up in production, you were warned.

first, I've created a reference the the Chart instance in a scope that is accessible to the chart configuration. I added the following tooltip configuration with filter function. It's probably best to check first if there are any active tooltips before grabbing the index.

tooltips: {
          mode: 'x',
          intersect: false,
          filter(item: Chart.ChartTooltipItem, data: Chart.ChartData) {
            const currentIndex = lineChart.tooltip._active[0]._index;
            return item.index === currentIndex;
          }
        },

Again, both _active and _index are private variables so they may change at any time.

[etimberg]

So part of this is how mode: 'x' works. It finds all the things that pass a hit-test check. If multiple points overlap, they will all be returned. A point is considered in range if the distance(point.center, mouseEvent) < point.radius + point.hitRadius hence why multiple points are found here.

The filter work was done in #7556 and there is a better mode to use to achieve this behaviour. If the mode is set to 'nearest' and then the axis is set to 'x', the single nearest item along the X axis from the event position will be found. See https://codepen.io/etimberg/pen/bGepzqy

Between the filter and the better mode, I am going to call this resolved.