Each plot represents 8 bars of music. The red dots show the actual tempo for each beat measured fairly accurately directly from the audio file. The black middle curve represents the average of 20 tapping trials trying to identify the tempo. The light gray outer curves are the minimum and maximum tempo values tapped during the 20 trials. The dark gray lines in between each of the outer lines and the central line is the 95% confidence level of the true average of the 20 tapping trials.

When the red dots are outside the range of the light gray lines, it is probable that this represents a sort of surprise for the listener, or in this particular case the person tapping to the music. Even after listening to the performance for twenty times, the tapper could not match the tempo around those particular beats.