Charting cumulative date-series data, that isn't in order in a worksheet

So, I have sales folks entering sales opportunities into worksheets, but they are NOT in date order.

I wish to create a chart the shows the cumulative sales totals, accumulating them in date order.

I know how to do this IF they sales data is entered in date order.

But if is not, I must either re-enter it chronologically, or sort the worksheet chronologically, and then create a column that performs the cumulative values.

However, this approach means I must re-sort the values EVERY time the sales person enters his sales opportunities.

the Excel Charts allow the use of a time axis that will automatically sort and plot values in chronological order.  But I need the charting to perform the cumulative function as well.

Ideas?