How to use Monday-based weeks as timeUnit?

[bmesuere]

I'm creating a graph where I aggregate data by week. I do this by setting the timeUnit of a temporal field to yearweek. The aggregations works fine, but vega-lite (and vega?) seem to use Sunday as the first day of the week.

Is there a way to easily use Monday-based weeks instead (as per ISO standard)? Right now I'm shifting the dates by one day in preprocessing which is a bit hackish.

[jheer]

Unfortunately, I think this will require extensions to Vega itself, starting with the vega-time package: https://github.com/vega/vega/tree/master/packages/vega-time

(PRs welcome!)

[bmesuere]

I can look into creating a PR next week. Is adding an isoweek time unit the way to go?

[jheer]

Thanks! Yeah, something like that is probably the right path. d3-time uses timeWeek for Sunday-based weeks, then provides timeSunday, timeMonday, timeTuesday, etc. for shifting the week start day.

I'm not sure if we would want isoweek or a different (perhaps more descriptive?) name. We could always revise naming before release once an implementation is in place. A more pressing question is if we want to add new varieties of week time units, or add a separate parameter to specify the week start day (e.g., 0 (default) for Sunday, 1 for Monday, following the JS Date.getDay() numbering), similar to how we specify UTC vs. local time.