This calendar does not only aim to show the waves of the tide, but to give you a sense of the planetary relationships, gravitation, and the circular nature of time.
The height of the tide is based on predictions made by the Port of London Authority (http://www.pla.co.uk/assets/platidetables2017.pdf).
The outer circle is where the surface of the earth is, and the mean sea level. The tide has high points and low points from there, usually two of each per day. The sun and moon are meant to show the heightened combined gravitational effect they have on the earth during new and full moon.