When you set a grouping variable (for example you might Barchart Sales grouped by Region) the default is to sum the data into each category defined by the group code. This will give one bar for each unique value in Region, each bar being the total sales for that region. The default is to make one ‘bucket’ for each Region, sorted alphabetically.

If you want to preset the buckets – maybe you don’t have data for all of them – use this property to set them. This will also set the ordering, which is essential for monthly data.

ch.Bar yield GroupBy year GroupInto 1930 1931 1932
View ch.Close 

You can split the data with a category variable as well as grouping it, which effectively gives you 2-way accumulation as the category split is done first, then the split data is grouped.

See also ...
GroupBy – group variable