I would like to set something different insted of that 12K (on x axis)
I think this is not possible at least through reading the code and debugging the library.
Only way I can set custom value instead of this 12k is to change library (method createHoverText)
you can try using
See example: http://codepen.io/etpinard/pen/PGgQxR
And you might look at http://bl.ocks.org/zanarmstrong/05c1e95bf7aa16c4768e to help out with the number formatting language we use