throughput: data_len belongs to the next time slot should be added to
self.data after the previous time slot is processed
throughput: Unit conversion is missing in the final summary output
throughput: add an option --per-second (-p) to normalize values and graph

also fix a typo in the description of --mode.
throughput: fix indent length

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>