[monit] Alert on low swap?

[Simon Blandford]

Is there an easy way to trigger an alert when a system runs low on swap 
memory?

In the documentation I can only find tests for RAM (memory & total 
memory) but nothing specifically for swap.

To illustrate what I want to test, from the "free command"

# free
            total       used       free     shared    buffers     cached
Mem:       3072000    2800152     271848          0     123548     457420
-/+ buffers/cache:    2219184     852816
Swap:      2031608     484992    1546616

It is the 3rd column of the "Swap:" row that I want to test.

If there isn't a way then I can write a script to interact with monit, 
perhaps via a log file, but I am wondering if I haven't perhaps missed 
something really obvious since available swap is a basic metric in 
system monitoring. I can't believe it's not there.