I was reading about the Circle map in this Wikipedia entry. This is a dynamical map with a single variable $\theta$, and dynamics defined by $$\theta_{n+1}=\theta_n + \Omega -\frac{K}{2\pi} \sin (2\pi \theta_n)$$
The entry features the following plot of the recurrence time as a function of the two parameters
Where $\Omega$ varies from 0 to 1 along the $x$-axis, and $K$ varies from 0 at the bottom to $4\pi$ at the top. The redder the color, the longer the recurrence time.
Where does the symmetry of the plot reflected along $\Omega = \frac{1}{2}$ come from? Is it something that is evident from the equation?