first I/O time graph in darshan-job-summary can be misleading if not all ranks perform I/O
File to use as a reproducer is attached, but the issue should be visible from any parallel application doing I/O from rank 0 (or any other subset of ranks). The first graph shows "Average I/O cost per process", which will look low on those apps because it amortizes I/O time across ranks that aren't doing I/O.
It would probably be more intuitive to show slowest_rank_time/run_time instead, so we show the percentage of time spent in I/O on the worst case rank rather than average across all ranks regardless of whether they participate or not.