But not using it as some linters are either plain incorrect (the one
suggesting to not use nil for `c.t.Context()`) or just
debatable (checking for err value is a good practice, but there are
good reasons to opt out in some cases).
This displays a line for the previous period on stacked graphs.
Previous period depends on the current period. It could be hour, day,
week, month, or year.
This is needed if we want to be able to mix use of several tables
inside a single query (for example, flows_1m0s for a part of the query
and flows_5m0s for another part to overlay historical data).
Also, the way we handle time buckets is now cleaner. The previous way
had two stages of rounding and was incorrect. We were discarding the
first and last value for this reason. The new way only has one stage
of rounding and is correct. It tries hard to align the buckets at the
specified start time. We don't need to discard these values anymore.
We still discard the last one because it could be incomplete (when end
is "now").
