NetSim plots the moving average of throughput for links and applications.
WIth regards to instantaneous throughput, at any point of time either a packet is being transmitted (or received) or it is not. Hence the instantaneous throughput would either be the Max_link_speed or it would be zero.
However, different users could have different interpretations for instantaneous through and the best way to derive it would be to use the packet trace file. More information is available in the "Results and Analysis" section of the user manual