Couple of days ago I had a problem that I couldn't solve after ~ 2 hours, and decided to ask on IRC. Almost immediately after asking, I figured out the solution, but David asked me to write about the solution, even though it's now (for me) completely obvious.
The problem was like this:
I had two tables, with very simple structure: event_when timestamptz, event_count int4, and wanted to show it as a single recordset with columns: event_when, event_count_a, event_count_b, but the problem was that event_when usually didn't match. Here is an example: