2 series trend line

[etamer]

Hi there, I have a scatterplot composed of two data series- meaning I have two different colors sets of data points. I need a single trend line that is graphed as if the two data series are one. I have looked around searching for a solution and heard the function 'SERIES' could be of use, but am unsure of how. Any ideas of how to use 'SERIES' to resolve this problem or how another way this could be done? 

[#2]

I think you should be looking into a different solution. A trendline is always tied to the series' data so if you need the trendline to be calculated against a different set of data, you would best use the existing worksheet functions to calculate the trendlines' parameters using e.g. the LINEST or SLOPE and INTERCEPT functions.