Please help me to correctly read the traffic graphs

[maxferrario]

Hi,
I recently realized that I'm not sure I correctly understand "interface - traffic" graphs.

When I look at a graph for a short duration (less than 48 hours, using the presets) I see something like that (which is easily readable even for a dyschromatic person like myself):

cacti-bandwidth-graph-1.png (61.65 KiB) Viewed 652 times

The graph only shows two sets of data, rendered as "bars", namely the inbound and outbound traffic.

But when I display the graph for a longer period of time four data sets are plotted:

cacti-bandwidth-graph-2.png (79.56 KiB) Viewed 652 times

Looking at different periods, it seems to me that the "line values" are not averaged.

My question is: do those two lines on the graph represent the maximum values of the two data sets?

Thank you very much,
Massimo