I have data where for some of the series there is data in each "category" and
in other series there is some missing data. Here's the example:

Fund Type Fund Company One Year Return
Target 2050 Principle 12.2
Target 2050 TRP 11.3
Target 2045 Principle 11.1
Target 2040 Principle 10.8
Target 2040 TRP 9.4

When I chart this I get a line for all of the "Principle" data because it
has a data point for all three "Fund Types". I only get data points and no
line for "TRP" because it has no data point for the "Target 2045" "Fund
Type".

I would like to add a trend line that simply "connects the dots", but none
of the available trendlines will do that. I also can't put a formula in the
cells because the source data is a pivot table. Any solutions for this?