If stopping potential and frequency of incident radiation be respectively plotted along verticle and horizontal axes, for different photosensitive substances, then which is same for all curves?

Stopping potential  $V_s=\left(\frac{h}{e}\right)\nu -\frac{{\varphi }_0}{e}$, where $\nu$ is the frequency of the incident radiation and ${\varphi }_0$ is the work function of the substance.

Obviously, it is of the form $y=mx+c$

where slope  $m=\frac{h}{e}$ is a universal constant.