Using crank angle-resolved data in Matlab/Simulink

Hi, I'm trying to analyse some cylinder pressure data that is crank angle-resolved, i.e. asynchronous in time. I have one sample every 0.5 degree.

So I have two questions:

1. How do I use e.g. Butterworth filter ( butter() ) on the pressure data in Matlab when I do not have a constant sampling frequency?
2. How am I supposed to handle this in Simulink, discrete data without a constant sampling frequency?

EDIT: I should add that I have no time vector, just a vector of the according speed of the engine in RPM. Regards, TheNovice