Illustrative photo

A good way of explaining at the same time the meaning of the x-axis, and  how the pattern shown in the graph looks like in the real world. Note that the lower feather width in primary 8 is easily appreciated in the picture.

Reference Hingee, M. and Magrath, R.D. 2009 Flights of fear: a mechanical wing whistle sounds the alarm in a flocking bird. Proceedings of the Royal Society of London B published online 2 September 2009 doi: 10.1098/rspb.2009.1110 (Journal website)