score:2

Accepted answer

I don't know about the clip path, but this fixes the area so it doesn't descend below 0:

var area = d3.svg.area()
    .interpolate( "monotone" )
    .x(function(d,i) { return x(i); })
    .y0(-y(0))
    .y1(function(d) { return -y(d); })

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