Maybe you could create a field which holds a composite of arrival time for all pumpers and for the first ladder per incident.
LOAD FirstSortedValue(RowID, ArrivalTime) as RowID, FirstSortedValue(ArrivalTime,ArrivalTime) as PL resident Arrival where Type = 'Ladder' group by Incident;
LOAD RowID, ArrivalTime as PL Resident Arrival where Type='Pumper';
left join (Arrival) LOAD * Resident TMP;
drop table TMP;
Then do your firstsortedvalue(PL,PL,3) in the chart.
see also attached,
P.S. This is not selection sensitive.
DEV Sandbox1_SW.qvw 148.0 K