to be clear, they do want them counted in more than one place? so a 5 minute row is counted in both <15 and <60...and they want to see duplicated data inside the same total % chart?

I'd lean towards maybe a union statement for the data source to create the multiple rows as that are needed for this multiple grouping or maybe go with a sub-report for only the chart.

No problem.
If I think of something else I'll post it, or maybe someone else will chime in. Good luck.

My report prompts for a start and stop date. Based on the difference between those days, different dates are output.
So if a week or less, I report each day;
under a month, I report each week;
under a year, I report each month;
over a year, I report each year;

so here's the example:

        if datediff('d',{?Start Date},{?End Date})<8
            then totext(date({Command.CALL_INSTANT}), 'MM-dd-yy')
        else if datediff('d',{?Start Date},{?End Date}) in [8 to 30]
            then totext(dateadd("ww",datepart("ww",{Command.CALL_INSTANT})-1,date(year({Command.CALL_INSTANT}),1,1) - dayofweek(date(year({Command.CALL_INSTANT}),1,1)+0)), 'MM-dd-yy')
        else if datediff('d',{?Start Date},{?End Date}) in [31 to 365]
            then totext(date(year({Command.CALL_INSTANT}), month({Command.CALL_INSTANT}),1), 'MM-dd-yy')
        else if datediff('d',{?Start Date},{?End Date}) > 365
            then totext(date(year({Command.CALL_INSTANT}),1,1), 'MM-dd-yy')

if you wanted it to do it by month, you'd care about this portion:
        else if datediff('d',{?Start Date},{?End Date}) in [31 to 365]
            then totext(date(year({Command.CALL_INSTANT}), month({Command.CALL_INSTANT}),1), 'MM-dd-yy')

but I'm only using the code above because I want things to change based on how many days of data I'm pulling, if you're just using a date/time column, you can have it display by month by going into the chart expert and under columns if you have your date highlight it and choose group options and then under "The column will be printed", choose "for each month".