Scatter Plot of OQPSK
You'll need to plot your signal's imaginary and real (I and Q) part at the exact symbol time – in other words, you'll need timing recovery to get the constellation from a time signal.

Timing recovery is among the more complicated things you'll need to implement for a receiver, depending on the actual kind of transmission you're dealing with. 

Assuming you weren't aware of that, you might want to  look into these terms (I think the order makes sense, didactically):


pulse shaping filter
eye diagram
Nyquist ISI criteria
timing recovery


square timing recovery
Gardner timing recovery

phase error correction


Costas loop