The from/to range of the data that the info popup is showing is given as the third argument (it is an array with two values).
The example shows the basic use of these values.
If you want to format the values exactly as the chart does it by default it will be a bit harder. The formatting is done with moment, the intended scenario is for you to include your own version of the library however it is unlikely that until v2 we would change the fact that you can retrieve it by using ZoomCharts.Internal.moment
.
Another piece of the puzzle you might need is the displayUnit()
method that will tell you if the chart is showing months, years, etc.