The Stability Region for Runge-Kutta

The stability region for standard Runge-Kutta is shown below. It is the interior of the curve |1+z+z²/2+z³/6+z⁴/24| = 1.

Close up near the origin:

Curve intersects the negative real axis at -2.7853 (approximately). And the imaginary axis at (plus or minus) 2.8284i (approximately).