Lowess smoother: Robust locally weighted regression.
The lowess function fits a nonparametric regression curve to a scatterplot.
The arrays x and y contain an equal number of elements; each pair
(x[i], y[i]) defines a data point in the scatterplot. The function returns
the estimated (smooth) values of y.
The smoothing span is given by f. A larger value for f will result in a
smoother curve. The number of robustifying iterations is given by iter. The
function will run faster with a smaller number of iterations.
x and y should be numpy float arrays of equal length. The return value is
also a numpy float array of that length.
e.g.
>>> import numpy
>>> x = numpy.array([4, 4, 7, 7, 8, 9, 10, 10, 10, 11, 11, 12, 12, 12,
... 12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 16, 16,
... 17, 17, 17, 18, 18, 18, 18, 19, 19, 19, 20, 20, 20, 20,
... 20, 22, 23, 24, 24, 24, 24, 25], numpy.float)
>>> y = numpy.array([2, 10, 4, 22, 16, 10, 18, 26, 34, 17, 28, 14, 20, 24,
... 28, 26, 34, 34, 46, 26, 36, 60, 80, 20, 26, 54, 32, 40,
... 32, 40, 50, 42, 56, 76, 84, 36, 46, 68, 32, 48, 52, 56,
... 64, 66, 54, 70, 92, 93, 120, 85], numpy.float)
>>> result = lowess(x, y)
>>> len(result)
50
>>> print("[%0.2f, ..., %0.2f]" % (result[0], result[1]))
[4.85, ..., 84.98]
 Returns: yest
