I'm trying to figure out how to do the following when the above sales chart can be dynamic, meaning months can be added to the end, or even at the beginning, as more sales data is entered. If the user inputs a salesperson's name (Chris in the above example) and the range of months that they want sales totals for, it will return the sum of that range (275 + 375 + 425 + 500 in the above example) AND all sales through the end date starting with the very first month in the chart, whatever that month may be (400 + 265 + 275 + 375 + 425 + 500 in the above example). I've tried to do this using INDEX and MATCH, SUMPRODUCT, SUMIF and INDEX, SUMPRODUCT etc. but I can't quite get it to work. I've even tried using a helper row that returns the column number (using MATCH) of the chosen begin and end months, so I'm not opposed to using a helper row (above the year row for example) if that makes it easier to manage.
Is there a way to do this? Any help is greatly appreciated.