Vega supports arrays for interval time units. For example ["year", "month", "date"]. In Vega-Lite, we support strings yearmonthdate since they are easier to type and validate. However, it may make sense to support the array syntax. One limitation of our string approach is that we don't support all combinations of time units (e.g. yearseconds). We don't want to just support string since we would lose the ability to validate. However, adding the array syntax would introduce another way to specify interval time units.

@kanitw, @arvind and @haldenl probably have opinions here.