Interface Availability graph Issue

I am monitoring interfaces in SolarWinds NPM. Availability graphs of these interfaces show gaps. I couldn't understand why it is so? Its behavioure is very strange if i select the time period from 9AM to 11AM and sample interval is every minute then it shows a single bar at the start of each hour instead of showing bar for every minut in the hour. if i select 5min then that single bar expand to 5 min in each hour. Same with the 10, 15, 30 and 1 hour sample interval. I am attaching snapshot of graphs for sample. Why it is showing this behaviiour kindly tell me if anyone knows.