function Regression(params) {
    Transform.call(this, null, params);
  }
Compute regression fits for one or more data groups.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.x - An accessor for the predictor data field.
@param {function(object): *} params.y - An accessor for the predicted data field.
@param {string} [params.method='linear'] - The regression method to apply.
@param {Array<function(object): *>} [params.groupby] - An array of accessors to groupby.
@param {Array<number>} [params.extent] - The domain extent over which to plot the regression line.
@param {number} [params.order=3] - The polynomial order. Only applies to the 'poly' method.