Inconsistency in unsorted timestamps behaviour

[si14]

When using on 2 separate files (one of them contains numeric track and other one contains events)

si14@si14-laptop:~/work/analytics/pings-12-12
> cat data.txt tasks.log | tplot -dk "lines" -k task "event" -if - -o "temp.png" -or 1400x800 && eog temp.png   
Warning: working in non-streaming mode (for very large datasets, supply input from a file)

(eog:8277): EOG-CRITICAL **: eog_list_store_length: assertion `EOG_IS_LIST_STORE (store)' failed

si14@si14-laptop:~/work/analytics/pings-12-12
> cat data.txt tasks.log | tplot -dk "lines" -k task "acount 5" -if - -o "temp.png" -or 1400x800 && eog temp.png
Warning: working in non-streaming mode (for very large datasets, supply input from a file)
tplot: Times are not in ascending order

I haven't found anything about ascending order in docs.