Not sure how meaningful such a scheme would be over time. If a community, such as here, is volatile then user could be interacting with posts/threads/etc on a daily basis. In other equally active cummunities I am running at over 100 days straight and approaching 200 overall. The long list of numbers from such visits would be produce someting akin to a random number generator.
Even a plot might not be helpful for such communities and regular users as the graph would be ever growning. Even a Tufte sparkline might not help. One might be able to infer from some data points that the user has been on vacation but how much more useful would it be.
The aggregate numbers of present may prove to be the most useful way of displaying the data.