The graph between frequency $\mathrm{\upsilon }$ of $k_{\alpha }$ x-rays and the atomic number Z

We know

$\sqrt{\mathrm{\upsilon }}=\mathrm{a}\left(\mathrm{Z}-\mathrm{b}\right)$

where, “a” represents the proportionality constant, “b” represents the screening constant and Z is the atomic number of the element.

Squaring both sides, we get

$\mathrm{\upsilon }={\mathrm{a}}^2{\left(\mathrm{Z}-\mathrm{b}\right)}^2$
So the graph should resemble a parabola  $(y=x^2)$