Trendline for only part of data?

I have a scatter plot with two sources of y data using the same x data (i.e. y1(x) and y2(x) are plotted for the same range of x), and I have a legend to say what each graph is. I want to make a trendline for the first half of y1, but no the whole range, but whenever I use the graph's trendline option, it automatically does it for all of y1. I also tried breaking up y1 into two separate data sources, lets call them y1a and y1b, and used the trendline on just y1a, but then my legend had three labels -- y1a, y1b, and y2 -- and I only want two (y1, y2). Is there any way to make a trendline for just part of y1 without changing the legend?

Would it be a satisfactory solution to simply delete the y1b legend entry (select the legend-> select legend entry -> delete)? Then you'll only have two legend entries.

calculate the trend line information yourself using the slope and intercept on only the data you are interested in.

By the way, I have a lot of knowledge about natural variation in data, and trend lines are meaningless, if applied to current data, if the process producing the data is stable. This is because the precise value of each point is random within +- 3 standard deviations of the mean. However, where the process is unstable, eg 21,18,15.1,12.2,9,6.1,3 you can clearly draw a line on the data to represent the current trend.


If you are just taking a chunk of historical data, and only want the trend for that time, regardless of the state of the process producing that data, then that's fine.