How to get a Trendline to Ignore Blanks in Data

I've viewed a few threads on this, didn't find an elegant solution. Does anyone have one? I've attached an example that I'll explain.

I need to break my data sets up by 8 week periods. I need trend lines for each period. The second period starts in the next 8 weeks. I can get the data to only show for the second set of 8 weeks, but the trendline wants to start from the beginning. How do I make the trend line cover the same period as the second 8 weeks?

Andy, that is perfect, thank you.

Funny, I actually ended up on your site yesterday when doing a Google search about this topic.