In d3, how to get the interpolated line data from a SVG line?
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I display a line chart with D3 with roughly the following code (given the scale functions `x`, `y` and the float array `data`):

     var line = d3.svg.line()
             .interpolate("basis")
             .x(function (d, i) { return x(i); })
             .y(function (d) { return y(d); });
     d3.select('.line').attr('d', line(data));

Now I want to know the **vertical height of the line at a given horizontal pixel position**. The `data` array has lesser data points than pixels and the displayed line is **interpolated**, so it is not straight-forward to deduce the height of the line at a given pixel just from the ``data`` array.

Any hints?
