Line graph for cumulative number over time

[Jane]

I have a Problem Register with a single row entry for each problem raised
and, eventually, fixed. There is a Date Raised and a Date Fixed column.

I would like to display a line graph with cumulative no. of problems raised
over time and, ideally on the same graph, cumulative no. of problems fixed
over time. The Date Raised column I would expect to be in ascending order
but the Date Fixed column will be "random". As time progresses, one can see
the Fixed line closing in on the Raised line and eventually meeting.

eg
Prob Date Raised Date Fixed
1 2-JAN-2006 5-JAN-2006
2 2-JAN-2006 2-JAN-2006
3 3-JAN-2006 4-JAN-2006
4 4-JAN-2006 5-JAN-2006

I'm not getting very far on my own. Any suggestions?

Thanks,
Jane