tooltip.format.value index variable different on Chrome vs Safari/Firefox

It seems I have stumbled across a bug with the tooltip.format.value behaviour. See [JSFiddle](http://jsfiddle.net/h2ud3cxy/)

I've added a tooltip to return the `index` of a datapoint on mouseover. In Safari and Firefox I get the same value for the index variable but on Chrome I get different values.

Any help in this regard will be greatly appreciated.

Start by reproducing the linked JSFiddle in Chrome, Safari, and Firefox and compare the index passed to tooltip.format.value. The issue names no source file or test; done means the browser discrepancy is resolved or its cause and expected behavior are clearly documented.

Written by the indexing model from the issue text.