Labels not rounded to necessary precision

[LondonWebFactory]

I have found a labeling bug.

Version
https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.2/Chart.bundle.min.js

Browser
Google Chrome
Version 67.0.3396.87 (Official Build) (64-bit)

CodePen
https://codepen.io/LondonWebFactory/pen/VdyEmp

Description
Hover over elements 3, 6 and 7 and you will see that there is an error in the data. For some reason chart JS is adding 1 times ten to the power minus 16 (or thereabouts - it's hard to count)

[LondonWebFactory]

A fix can be found in this post:

#4992

[benmccann]

This should be fixed in 2.7.2 by #4943

@jcopperfield any chance you would be able to look at this one? It seems your fix for rounding to the necessary precision might not be working in all cases

[jcopperfield]

@LondonWebFactory @benmccann As far as I understand the requested problem lays within the tooltip not the axis.

The solution for this problem is in the tooltip documentation. It is actually the exact example demonstrated in this page.

var chart = new Chart(ctx, {
    type: 'line',
    data: data,
    options: {
        tooltips: {
            callbacks: {
                label: function(tooltipItem, data) {
                    var label = data.datasets[tooltipItem.datasetIndex].label || '';

                    if (label) {
                        label += ': ';
                    }
                    label += Math.round(tooltipItem.yLabel * 100) / 100;
                    return label;
                }
            }
        }
    }
});

[benmccann]

Ah, thanks @jcopperfield. I missed that your PR only affected the axis. Would it make sense to make the change you made on the axis affect the tooltip as well?

[jcopperfield]

@benmccann Well it could be implemented in the default label callback in core.tooltip.js. I'm not sure it is worth to make it the default behaviour, as it might lead to incorrect behaviour for other types of axis. I think the solution as described in the documents is simple enough to implement and customize.
It might be better to ask @etimberg for his opinion though.

[etimberg]

Not sure why I didn't see this last june, but the issue here is the data going in.

var x = 3 * 0.1;
console.log(x);

0.30000000000000004

This is caused by using floating point numbers. I think the solution of using the label callback is the correct one since it has the most flexibility and we don't want to start making decisions on what the numbers mean