Or, if you are already using cacti traffic data for that interface in your map (e.g. for a link connected to this node), then you can either use the dsstats: plugin or enable poller_output support ("SET rrd_use_poller_output 1" at the top of the map file). Both of those take a couple of polls to get going, but after that they will automatically populate a bunch of data as new variables, taken from the Cacti snmp cache.
In theory, it should run a bit faster than reading rrd files the normal way, too.
There's more description of the variables and poller_output mode in the manual: http://network-weathermap.com/manual/0.98/pages/targets.html#rrd
So after that, you'd be able to use:
where relatedlinkname is the link that is using the traffic stats for the relevant interface.