Numbers: Grouping daily data into weekly data
Figured this out, finally. Thanks, community!

I created a table with weekly dates. Then I added summary columns for the daily data:

=SUMIFS(
  column_I_care_about_from_daily_spreadsheet,
  date_field_in_daily_spreadsheet, "&gt;="&amp;date_in_this_spreadsheet, 
  date_field_in_daily_spreadsheet, "&lt;"&amp;date_in_this_spreadsheet
)


I then added a "summary" column to match the weekly data

=SUMIF(
  date_field_in_daily_spreadsheet,
  "="&amp;$date_in_this_spreadsheet,
  column_I_care_about_from_daily_spreadsheet
)


Two things:
- Yes, the second formula is overkill
- Yes, SUMIF and SUMIFS reverse their fields from each other

From here, I could easily get a graph.