code: 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
docs: 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