Only reports allow you to compute available statistics to display trends.
Advanced Alert can only use available stats without being able to modify them. This looks like a feature request.
Alternatively, you could use UnDP and Transforms pollers to compute trends. Then you can configure a an alert on the result of the Transform poller. This however means devices are going to be polled for the same information twice only to support this feature.
The best option remains to use reports for now.
HTH,
Yann
