thanks for reply.
as per the requirement count of exam id should include all exams that are running at that specific minute
for eg consider the below data set
examstarttimeutc examendtimeutc 5/2/2016 7:19 5/2/2016 9:37 5/2/2016 7:32 5/2/2016 9:25 5/2/2016 8:28 5/2/2016 9:52
so when i plot the graph at 8:28 the count should be 3
as the above two exam are also active at 8:32
basically we have to plot total no active exam at every minute