How can I trim a trend line in a graphic?

[mlarruda]

Well, I need to to a chart of, say, sellings by month, splitted by period. I managed this by using two dimensions: MonthName(DATE) and

=IF(DATE <= 'xx/xx/xxxx', Dual('before', 1), Dual('after', 2))

Now I need to get the trend line of each period. I managed it by clicking in this box, from the Expressions tab.

The problem is that the lines are spanned all over the width of the graphic and I want to trim them, letting one line ending in the month xx/xxxx and the other starting in the same month.

Is it possible? Many thanks in advance.