For the graph to intersect y=x line. The value of |v| and |u| must be equal. From lens formula
1
v
−
1
u
=
1
f
When u=−2f,v=2f Also v=
f
1+
f
u
As |u| increases, v decreases for |u|>f. The graph between |v| and |u| is shown in the figure. A straight line passing through the origin and making an angle of 45° with the X-axis meets the experimental curve at P(2f,2f).