D3 multi line chart mouseover issue

[Arnav Bose]

I found one solution in jsfiddle: 

http://jsfiddle.net/takuan/gakdeL1u/7/

Here, the mouseover is actually working simultaneously for both the charts. I am stuck at this point as I want mouseover to work for only one chart - i.e. the chart over which I am hovering the mouse. Could the same code be used to get the effect I am looking for? 

[Николай Слободчук]

Here's an advanced solution http://bl.ocks.org/mbostock/8033015 which I'm trying to digest myself.

If you have only two lines I guess you could divide the area of the chart by calculating the invisible line which the average of the two lines.