Constellation diagram shows two things at once - amplitude in decibels and phase of the selected frequency bin. If point is far away from chart origin it means that signal is strong, if near origin it means that signal is weak. Position of point on the circle tells about phase.
In this example phases are displayed as hands on clock.
At the top (12 o'clock) we have phase offset equal to 0 degrees (or 360 degrees since it's the
same). Values goes clockwise so point on the far right side will have 90 degrees phase offset
(3 o'clock), point on the far bottom will have 180 degrees phase offset (6 o'clock) and so on.
To test that please select frequency bin with peak amplitude and try to manipulate phase of that sine from 'Separate sines' section.