Plotting NA() or #N/A data points

I have formulas that return the NA() or #N/A to cells I used for plotting on
charts. The last two cells do not show up on the chart, as intended. However,
when I enter the data into the first of the two cells, it plots the last data
point as zero.
Ex: Jan-Oct has data, Nov and Dec default to the above. When Nov is filled
in, the chart plots Nov and then adds Dec at zero value and returns #N/A in
the axis.