To do what you want, you can first "calculate" it visually - by using the IO Graph. Load your file and then Select Statistics:IO-Graph. You might want to apply one or two appropriate display filters in the Filter box. Then if you are say interested in the bits per second in *every* second, set the Tick Interval to seconds, and Units to Bits/Tick. The Y axis heights at each Tick are your samples in bits per second. You can adjust the Tick Interval as you need, allow aggregation of that data at different intervals. (Obviously if your tick interval is say 0.1 secs, the height of each sample is bits per 0.1 seconds - for need to multiply by 10 to get a bps rate for each 0.1 second interval.
If I want to accurately determine the size of a particular burst, I try and find the first and last packet that contains the burst I want. I then define a display filter like "frame.number >= 123 && frame.number <= 456". You can then use the Statistics:Summary menu and look in the Displayed column. You can then use the appropriate calculated data, like time between first and last packet, bytes, or just Mbits per second
