Here is a test code to calculate the derivative of a curve in sliding
windows. I generated a linear example, and it should plot the derivative in
a straight line. But, it does not?!

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x = seq(0,10,by=0.1)
y = x * 0.5 + 0.1

window=15;
half = floor( window/2)

x2 = half: (length(x)-half) ;
slope = half: (length(x)-half) ;

count = 1;
for( i in half: (length(x)-half) ) {
   pos = (i-half):(i+half)
   m = lm( y[pos] ~ x[pos])
   x2[count] = x[i]
   slope[count] = m$coeff[2];
   count = count + 1;
}

plot( slope ~ x2, type='l' ) #does not give a straight line, why?

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