def fit_exponential(xs, ys)
      a, b, rr = fit(xs, ys, tran_y: ->(y) { Math.log(y) })

      [Math.exp(a), Math.exp(b), rr]
    end
def fit_exponential(xs, ys)
      a, b, rr = fit(xs, ys, tran_y: ->(y) { Math.log(y) })

      [Math.exp(a), Math.exp(b), rr]
    end
Find a line of best fit that approximates exponential function

 Model form: y = ab^x

 @return [Numeric, Numeric, Numeric]
   returns a, b, and rr values

 @api public