I'm not aware of this having been implemented in any other analytics package, but there is a very nice feature in a program called Cuterank that I think would really do a lot for Piwik users. The feature is annotations, associated with dates. In Cuterank, this is useful for tracking the effectiveness of one's SEO efforts. In Piwik it would help to keep track of anomalies. For instance, you might annonate a traffic spike "Digg Homepage Feature" or a flat period "Tracking code malfunction." I'm envisioning the annotations coming up when you hover over the date in a graph or report.