Line chart data series

Hi, I am sure this is simple but I need you experts out there. I have a data series containing cells with a formula to fill the cell depending on the date. Therefore up to the end of the year there will always be cells with a formula but no result. When I chart this data the chart drops to zero when it gets to the first "no result" cell. Have tried making the result "" i.e. blank but the graph still looks as if the poor salesman's figures have dropped to zero. What can I do with the data series to make the line on the chart stop at the last cell with data in it, rather than drop to zero. Obviously I could delete the formula in the cells representing future months but then I would have to put the formula back in each month.
Any help appreciated, thanks.
Jonathan