When I trend an eVar with Instances or a custom event as my metric, the graph is shown in terms of a Percentage. However, if I switch to a calculated metric, the graph is shown in terms of a Count, rather than Percent.
I've actually started just duplicating my events as calculated metrics so I can view them in a standard Count graph rather than Percentage. See the screenshots to see the difference.
It seems strange that Omniture would handle the graphs differently just because the metric is Standard vs Calculated. This would be a nice addition as a Graph Option, as I'm sure there are some good uses for the Percentage graphs.