Interesting - but very very odd.  COBALT is apparently not behaving anything like as designed.

As an immediate point, the first drop occurs 2 whole seconds later than either Codel or PIE, which is completely at odds with the excellent control of the initial slow-start phase seen in other graphs.

I took the timestamp files, dumped them into a spreadsheet column each, and plotted 1/(T(n)-T(n-1)) for each drop event against the raw timestamps.  This yields a view of the instantaneous drop frequency.  Because PIE sometimes drops multiple packets at once, yielding very high values here, I truncated the Y-axis at 1000 Hz.

The Codel implementation shows the expected behaviour of a linear ramp of drop frequency over time during its dropping phases.  The COBALT implementation does not.  Indeed, during a single phase, COBALT's dropping frequency appears to vary chaotically, as if it is implementing random-drop instead of timed drops.

 - Jonathan Morton