x-y scatter graph and confused
 

I am using Excel 2007.
I am trying to do a combination bar graph/ x-y scatter graph.
I have headings and data for date, last year sales, and this years sales.
Dates are formatted as 03/04/08 and sales are formatted for numbers to 1
decimal place.
Everything plots fine and "last years sales" are represented by the column
bar graph.
I have changed "this years sales" to a x-y scatter graph (lines connected to
the dots).
That combination plots fine.
On the x-y graph, I changed the secondary axis to primary axis.
I then save the workbook.
When I open the workbook, the graph has disappeared.
I can select the graph,right click on the graph, change the type of graph to
"2D column" and the graph reappears as a column graph.
If I use a combination column/line graph, the graph appears everytime even
after saving and reopening.
What am I doing wrong?
Thanks