- BIG TOUGH BUG: tiers look at total length of each away mesage, across *all
days*. but they're used e.g. in 7-layer graphs to choose tiers for *just
weekdays* or *just weekends*. should use only the length of the away messages
spent during the week or weekend, respectively. the right fix would require
redoing calc_*() to be able to restrict to certain days, time spans, etc. 
- BUG: infrequent msgs show 1 across all 24hrs in the 24hr distribution graphs
- BUG: how to provide SPOT access to config
- TODO: redo msgs_by_time and times_by_msg to use Bunches instead of tuples
- TODO: delete lerp code?
- TODO: better error msgs for layer away msgs
- TODO: better error messages (e.g. bad config values)?
- TODO: add layer parsing from config file (0.6?)
- TODO: away message globbing (0.6?)


