In SHM, velocity leads displacement by $\pi/2$ and acceleration leads displacement by $\pi$. This means velocity is maximum when displacement is zero (mean position) and zero when displacement is maximum (extreme position). Acceleration is maximum at extremes and zero at the mean. Think of it as a phase ladder: $x \to v$ (advance $\pi/2$) $\to a$ (advance another $\pi/2$, total $\pi$ from $x$). The acceleration-displacement graph is a straight line through the origin with negative slope $-\omega^2$.