functions:smoothing:SMOOTH
Depending on the tension, the smoothed curve may not pass through
the original data points. The tension corresponds to the curviness,
and must be > 0. If it is close to 0, each smoothing function is
almost a cubic spline and the resulting curve is quite loose. If it
is large, then the smoothing is almost linear. The tension is the
current value of TENSION, which may be changed with the SET command.
syntax: SMOOTH( x, y, xout, w )
If m is the length of w, x and y, the goal is to achieve:
sum(i=1 to n) of ((G[i]-y[i])/w[i])^2 = s where G[i] is the
cubic spline function at x[i] and s is the spline tension.
As w[i] decreases, the spline fits the data point (x[i],y[i])
more closely.