def derivate(self,xdata,ydata, xlimits=None):
x=numpy.array(xdata, copy=False, dtype=numpy.float)
y=numpy.array(ydata, copy=False, dtype=numpy.float)
if xlimits is not None:
i1=numpy.nonzero((xdata>=xlimits[0])&\
(xdata<=xlimits[1]))[0]
x=numpy.take(x,i1)
y=numpy.take(y,i1)
i1 = numpy.argsort(x)
x=numpy.take(x,i1)
y=numpy.take(y,i1)
deltax=x[1:] - x[:-1]
i1=numpy.nonzero(abs(deltax)>0.0000001)[0]
x=numpy.take(x, i1)
y=numpy.take(y, i1)
minDelta = deltax.min()
xInter = numpy.arange(x[0]-minDelta,x[-1]+minDelta,minDelta)
yInter = numpy.interp(xInter, x, y, left=y[0], right=y[-1])
if len(yInter) > 499:
npoints = 5
else:
npoints = 3
degree = 1
order = 1
coeff = SGModule.calc_coeff(npoints, degree, order)
N = int(numpy.size(coeff-1)/2)
yInterPrime = numpy.convolve(yInter, coeff, mode='valid')/minDelta
i1 = numpy.nonzero((x>=xInter[N+1]) & (x <= xInter[-N]))[0]
x = numpy.take(x, i1)
result = numpy.interp(x, xInter[(N+1):-N],
yInterPrime[1:],
left=yInterPrime[1],
right=yInterPrime[-1])
return x, result
