first of all let me tell u what exactly a phasor diagram is.
It is a mapping in which a vector is represnted by a line having its length equal to its magnitude and the angle which it makes tells us the phase difference with respect to a refference vector represented by the horizontal axis.
eg. if V = v cos(wt + a)
then if i take X = x cos(wt)
as a reference vector then, the vector V will be making an angle of a with X
thus if V makes an angle with I in a positive sense then wee say that voltage leads the current