Excel 2007 : Charting of zero values

Hi and thanks in advance for any help that might be given.
I'm using the following formula to obtain the average of 2 numbers but to show as blank if one of the numbers is missing as in no value for future dates.

=IF(A20=0,"",average(A20:A21)).

Fairly simple stuff but when I use a line chart to display the values and trends, the lines drop to the bottom when they reach the data for the future dates. If I leave the cells blank it charts correctly, but as soon as I extend the formula down the column into the future dates, the problem occurs. This happens regardless of how I format the series to show zero and/or null values. This of course prevents the trendlines from showing future trends as they also drop to the bottom of the chart.

Any and all suggestions will be greatly appreciated.

regds, Norm