Sorry, missed the cumulative summing you wanted.
Hopefully you do not have to use a crosstab as you can easily get this using a running total.
Set up a group on the Date field set to group on the month (same as in your crosstab.
Suppress the group name field (right click the field, select Format field, common tab check Suppress).
In section expert suppress GH1 as Suppress blank section and Suppress no drill down the details section.
Place a the date field in the GF1 and change the format to "month Year" or whatever you want it to look like.
Create a running total called "Cumulative Sum" (or whatver you want).
Field to summarize is your amount field
Type of summary is SUM
Evaluate: for each record
reset: Never
Place this on your GF1 .
You should end up with
GH1 - Group name (suppressed as blank)
Details - Suppressed
GF1 - Date field (formated to March-09) Running total field
E.g.
Feb 09 $470
March 09 $820
April 09 $880