Empty cells plotted as zero.

[ColinC]

In sheet 3 there is a column of figures (col. C)which builds up over time. When a group of four consecutive cells all have values in them the values are averaged and appear in cell P9 in sheet 4

=IF(COUNTBLANK(C13:C16)>0,"",AVERAGE(C13:C16))

This is working O.K. I now want to plot these averages but have the problem of empty cells being plotted as zero instead of being ignored.

=IF(Sheet3!P9>0,Sheet3!P9,NA())

I have had this problem before and have received advice from this forum concerning the NA() function. It has worked O.K. on similar programs but not this time. Help please.

Colin.