where \(A\) is the area of \(cross-section\) of the wire.
\(Young's\,modulus,\,Y = \frac{{\left( {F/A} \right)}}{{\left( {\Delta l/l} \right)}} = \frac{{Fl}}{{A\Delta l}}\)
\(\Delta l = \frac{{Fl}}{{YA}} = \frac{{F{l^2}}}{{YV}}\) \((Using(i))\)
\(\Delta l \propto {l^2}\)
Hence, the graph between \(\Delta l\) and \({l^2}\) is a straight line.