How do you determine graded versus abrupt diode junction types?
The junction capacitance of a reverse biased diode is given by

$$C = \frac{C_0}{ (1 + V_R/ V_0) ^ m }$$

Where 

\$C_0\$: Capacitance at No applied voltage

\$V_0\$: Junction or built in potential

\$V_R\$: Applied reverse bias

\$m\$: Grading coefficient = \$1/2\$ for abrupt and \$1/3\$ for step graded junction

The rest is all math. Simply divide the two equations, one for 15 pF and other for 13.05 pF, and take the logarithm. The result is 0.33. So it is graded.