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I'm just getting my head around the zplane function. I'm not how the values of z and p are mapped onto the pole-zero plot. I thought zeros and poles were supposed to have both real and imaginary parts - as these are the axes of the pole-zero plot. But in the examples of zplane code given in the help documentation the z and p values are just single real values. Also, for a 4 pole/zero filter there are 5 values for z and also for p. Shouldn't this be four?