Read the documentation for mrtg (which is what ClearOS uses) at oss.oetiker.ch/mrtg/
Here is an extract of one configuration option which might be what you are looking for... The mrtg files are in /etc/mrtg
By default the graphs only contain the average values of the monitored variables
- normally the transfer rates for incoming and outgoing traffic. The following
option instructs mrtg to display the peak 5 minute values in the [w]eekly,
[m]onthly and [y]early graph. In the example we define the monthly and the
yearly graph to contain peak as well as average values.
If not start searching the net for other graphing programs that will run under Linux such as cacti etc