Terminal voltage 'V' of the cell is V = E Ir
E is the emf of the cell, r is the internal resistance of the cell and I is the current through the circuit.
So, V = Ir+E
Comparing with the equation of a straight line y = mx+c, we get:
y = V; x = I; m = r; c = E
Graph showing variation of terminal voltage 'V' of the cell versus the current 'I' is
Emf of the cell = Intercept on V axis
Internal resistance = slop of line.