D3, using all the same y values, the graph of the line below

Question

It is best to look at:

http://bl.ocks.org/3824854 (this is the fork block bobmonteverdes # 2070123 )

For some reason, when you remove the selection of a “cosine wave”, the sine wave (which I hard-coded is not a sine wave at all, but a straight line) falls to the bottom.

Any idea what is going wrong here, or how can I correctly make this schema in the case where the data has the same y values?