How to plot data from serial port against actual time in Simulink S-functions?

I am receiving data through serial port from a microcontroller in a custom S-function block. Microcontroller is sending data sampled data after every 100msec (but it's not constant) whereas my simulation time step is 1msec. Matlab is making a plot against simulation time that is first sample at time t=0 and second sample at time t = 1msec. But I want to make a plot with the delay equal to the incoming data rate.