function DotBin(params) {
    Transform.call(this, null, params);
  }
Dot density binning for dot plot construction.
Based on Leland Wilkinson, Dot Plots, The American Statistician, 1999.
https://www.cs.uic.edu/~wilkinson/Publications/dotplots.pdf
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.field - The value field to bin.
@param {Array<function(object): *>} [params.groupby] - An array of accessors to groupby.
@param {number} [params.step] - The step size (bin width) within which dots should be
  stacked. Defaults to 1/30 of the extent of the data *field*.
@param {boolean} [params.smooth=false] - A boolean flag indicating if dot density
  stacks should be smoothed to reduce variance.