Can someone explain this plot of the Orr Sommerfeld equation?

[Navier-Stokes]

This is a plot for understanding the stability of flows through the orr sommerfeld equation.
However, I find it really hard to understand. The plot by itself is a little weird, what is it trying to convey?

 

[boneh3ad]

I feel like I typically see these two axes flipped, but the image is a representation of the eigenvalue spectrum where $Im(\lambda)$ is a frequency and $Re(\lambda)$ is a growth rate (as generally $c$ is assumed to be complex, $c = c_r + ic_i$). If $Re(\lambda) > 0$ then that eigenvalue is unstable.