Trendlines for partial data

Alan_Berow  wrote: ``separate trendline for each day``

Without a concrete example (Excel file with data and charts), I don't know what that means.

But in general, the only way to have trendlines for "partial data" is to create a separate chart "series" for each subset.  With some effort, you can make all of the subset series appear to be part of one complete series, making the line and marker colors the same for all.

Then select each subset series and click to add separate trendlines, again making the line colors the same, if you wish.

If you need to "extend to other stratified data", you might consider creating a macro -- although that might be challenging, depending on the similarity (or not) of the "other data".