[SciPy-user] question about splines

[Darren Dale]

I have a question about the interpolate.splrep and interpolate.splev 
functions, which I am using to find the inflection point in some data. 
I am testing these functions with a sine curve, just finding the spline 
representations and then evaluating them:

from scipy import *

n=1000
xmin, xmax = 0,1
x = linspace(xmin,xmax,n)
ymin, ymax = 0, 2*pi
y = sin(linspace(ymin,ymax,n))

weights = ones(n)/0.00001 # 1/stdev for noisy data
derivative = 2
print interpolate.splev(x,interpolate.splrep(x,y,s=n,w=weights),derivative).max()

If I were using this tool correctly, the reported maximum would be 
very close to 1 for any derivative. derivative=0 does yield a good 
approximation. If derivative is not 0, the amplitude of the result can 
be off by orders of magnitude. The amplitude of the derivative 
curve is effected by the limits of x and y, which I don't understand.

I must have overlooked something, does anyone have a suggestion?

Thanks,
Darren