Relation Between Superposition Principle and Uncertainty
Principle

The red curve is the sum of the blue curves. The larger
the spread in wavelengths of the blue curves, the smaller the size of
the resulting wavepacket (red curve). Drag rectangle (below the
picture) back and fourth to change spread in wavelengths.

Moving and Spreading Wave-Packet

Notice that as the wave moves, the group velocity (velocity of the peak
of the wave-packet) is larger than the phase velocity (velocity of one
particular crest of the wave). Also, notice that after a while,
the front of the wave has shorter wavelength than the back of the
wave.

Two Stationary and Spreading Wave-Packets: 2-Slit Experiment