Step charts are a good idea. Handling missing data in general is a bit tricky and I haven't full thought through the problem.

The real-time charts are build with a timestamp component which, given a specific time granularity (second, minute, etc.), could be used to handle gaps. Basic charts might require an option to switch on/off such behavior.

In case you're interested, I've opened up a new issue in the repository: https://github.com/fastly/epoch/issues/64

Thanks for the great suggestion!