Welcome in the ridgeline chart section of the gallery. Sometimes called joyplot, this kind of chart allows to visualize the distribution of several numeric variables, as stated in data-to-viz.com. If you're looking for a simple way to implement it in d3.js, pick an example below.
In term of code, a ridgeline chart is simply a set of many density plots. Thus, starting by learning the basics of density chart is probably a good idea.
A selection of examples showing the application of the basic concept to real life dataset.