FIR magnitude and phase plots: separate pi flips from 2pi wraps

For plotting FIR frequency response, factor at the midpoint delay so H = e^(-jcw) A(w) with A real, then |H| = |A|. Phase is -cw plus a pi jump only where A actually changes sign. A zero of A that only touches 0, as in 2 + 2cos(3w), adds no flip, so its only jumps are 2pi wraps of -cw. When plotting with np.angle in matplotlib, set values to NaN at jumps and where |H| is near 0, or false vertical strokes appear and suggest the phase is defined at the zeros.