draw the I-delta graph

Here is the plot between angle of incidence and the angle of deviation.

i is the angle of incidence, e is the angle of emergence, A is the angle of the prism and ? is the angle of deviation.

The graph is a parabola.

The graph indicates that each prism material has its own unique minimum angle of deviation. The red circle on the graph corresponds to the coordinate of angle of incidence for which minimum angle of deviation is observed.