Fix a couple of issues in throughput module

I noticed some questionable behavior in throughput module and want to fix them.
1. The data_len of the first packet belongs to the next time slot is added to the data of current time slot.
2. The final summary includes user-supplied unit name, but the values are not converted.
3. Graphs generated by different "-s" values are not directly comparable because the values are not the average per unit time.