It would be useful to have a "blur" function to approximate kernel density estimation.

This function would accept the output from histogram, conceptually looking something like:

blur(histogram(data))

Original discussion in d3/d3-contour#7 (comment)

The implementation could be similar to blurX or blurY in d3-contour.

This article may be relevant: Convolve n Square Pulses to Gaussian.