Trapper item graph draws data for all time period if there is at least 1 point

For Zabbix traps if there is at least one point on the graph, then there will be the line to the left and right sides. 

It can be expected that graph should contain empty space in place where data is not available. (But we can't know when data wasn't available and when it just shouldnt be sent becuasu of item type) 

To avoid misunderstanding, add a note to the documentation