function fourierplot(X,dfmin,dfmax,fsmp)
  % Plots a Fourier filter or transform {X} in the range {dfmin.dfmax}
  % of discrete frequencies, with the 
  % true frequency (in Hz) on the horiz axis.
  % Assumes that the number of original samples is even
  % and that the sampling frequency is {fsmp} (in Hz).
  assert(size(X,2) == 1)
  N = size(X,1);
  assert(0 <= dfmin)
  assert(dfmin < dfmax)
  assert(dfmax <= N-1)
  F = (dfmin:dfmax)*fsmp/N;
  assert(size(F',1) == dfmax-dfmin+1)
  plot(F',X(dfmin+1:dfmax+1,1))
 