Before revision:  The idea is to numerically solve the  linear advection equation with square waves of different wavenumbers as initial solutions. Therefore, the exact overshoots after one step of temporal evolution  can be easily calculated, and shown as a function of  CFL number and  reduced wavenumber.
After revision:  The idea is to numerically solve the linear advection equation with an initial condition of a square wave characterized with different wavenumbers. After one step of temporal evolution can be easily calculated, and shown as a function of CFL number and reduced wavenumber.