How do I calculate a running standard deviation?

How do I chart the standard deviation of a count?

You can use OPAL to calculate a daily count, running median, and running standard deviation from a series of values.

timechart 1d, daily_count:count_distinct_exact(block), group_by()
make_event
make_col runningAvg:window(avg(daily_count), frame(back:7d)),
runningStdev:window(stddev(daily_count), frame(back:7d))