Problem with Excel charts

[DKerr]

I have created the attached chart I would like to remove the blue area between Aug 10 and Sept 10. So that the blue area start in September only. I have tried leaving the reference cells blank, put in text in the pre September cells and tried to put "=na()" in the cells all to no effect. The blue area is created using the Area chart type with everything else being a line graph. Is there a way to remove this or to use a line graph and shade the area between two line graphs to achieve the same effect?