|Purple on the graph is off line, and this can be short blips if a line loses sync or longer. Notes (pins) are often added to graphs to explain why a line is off line if we know, especially when we are investigating a fault. The notes on this graph told when the BT engineer arrived and left.
Central Heating''' causing drops. [http://wiki.aa.org.uk/File:Cqm-heating.png View a week of these graphs] Here there is interference as the central heating goes on, the drops are regular - twice a day, at the time the heating goes on. This actually highlighted a fault in the central heating for this customer as the graphs showed no drops for 2 days running!