Why A constellation diagram is a polar/IQ plot where each point represents one symbol the modulator can transmit. The angle of a point from the origin is its carrier phase and the distance from the origin is its amplitude, so the whole set of points shows every phase/amplitude combination the scheme uses. QPSK has 4 points (4 phases, one amplitude, 2 bits per symbol); 16-QAM has 16 points spread over several amplitudes and phases, carrying 4 bits per symbol. In practice the plot is also used to judge signal quality, since noise and distortion smear the points into fuzzy clouds.
Watch out It does not count carriers or error-correction bits; a constellation applies to a single modulated carrier, and FEC overhead is a property of the protocol, not the modulation plot. Frequency response would be shown on a spectrum display, not an IQ plot.
Constellation = I/Q dots: angle is phase, distance is amplitude, one dot per symbol.