Hierarchical tooltip configuration definitions

[tiagoroldao]

As tooltips are triggered on Elements, a hierarchical callback definition could be made - callbacks are called if defined in (in order of preference):

• the dataset
• the axis
• the chart
• the chart globals

The reasoning behind this feature is that while creating a complex chart with different needs for label formatting, I found myself doing this a lot:

chartOptions = {
    [...]
    tooltips: {
        callbacks: {
            label: function (tooltip, data) {
                // real implementation accounts for nonexistent object properties
                var datasetCallback = data.datasets[tooltip.datasetIndex].tooltips.callbacks.label;

                if (typeof datasetCallback === 'function') {
                    return datasetCallback(tooltip, data);
                }

                return tooltip.yLabel;
            }
        }
    }
}

[etimberg]

This seems like an interesting idea. Thoughts @tannerlinsley @derekperkins @zachpanz88 @simonbrunel ?