*intercept* will always be 0.0, the underlying linear
function simplifies to:

    y = slope * x + noise

>>> y = [3 * x[i] + noise[i] for i in range(5)]
>>> linear_regression(x, y, proportional=True)  #doctest: +ELLIPSIS
LinearRegression(slope=2.90475..., intercept=0.0)

zKlinear regression requires that both inputs have same number of data pointsry