The "Trend" is an attempt to put a number to the increase in activity for a given port.
Right now, I am comparing the last 24 hours to the last 30 days.
So if we see a rise in activity compared to the last 30 days, the trend is high.
The following formula is used to calculate the trend:
sqrt( (S-s)^2/s + (T-t)^2/t ) )
S: number of source IPs hitting this port last 24 hrs.
s: average number of source IPs hitting this port each day (last 30 days).
T/t: same for target IPs detecting scans on this port.