Trendline to ignore empty cells
 

I have a chart with current sales and future sales. In the future cells I
have =NA() to ignore them because they are empty until a future date.
Because of this my trendline does not fall to zero but the trendline still
shows.

Is their a way to make the trendline not show at all for those empty cells
but then show at the end for the total average sales?

Thanks for your help!