Generating QPSK Constellation Diagram
This is what I do when making such a plot for a QPSK modulation:

First I upsample the I and Q data so that I can see more of the trajectory between samples given (assuming you start with at least 2 samples per symbol otherwise, due to Nyquist, the information on the trajectory between symbols is not available, but as long as you have 2 or more samples, all the information of what happens in the first Nyquist zone is preserved):

Is = resample(I, 10, 1);

Qs = resample(Q, 10, 1);


and then I make a plot of I vs Q 

plot(Is,Qs)


I used a resampling increase of 10, but you could make it higher if desired. The result is the interpolated samples for the full trajectory during symbol transitions.