trendlines starting in manually decided dates

Hi there.

A picture is worth 1,000 words. An Excel sheet is worth 1,000 non-editable pictures.

Please read the yellow banner about sample worksheets, at the top of the screen. Act on its guidelines and post a SMALL sample sheet complete with an explanation and some expected results.

There is no sample file attached!!

I agree with Glenn, a sample file that helps us understand exactly how you like to interact with the spreadsheet will help us with solutions that you can use.

FWIW, Here's one strategy I see for doing this (assuming I understand exactly how you want to interact with the data in the spreadsheet):

1) I find that Excel's built in regression/trendline algorithms rarely work well (if at all) when the desired subset of the data for the regression is buried inside of the larger data set. I find that I almost always need some strategy for extracting the desired subset from the larger data set. Sometimes I'll use filters, sometimes I'll use lookups, and there are almost certainly other strategies, but one way or another I find it is usually necessary to somehow extract the desired data from the database. For now, let's assume that you can use a simple Autofilter step to do the "filtering." (https://www.wikihow.com/Use-AutoFilter-in-MS-Excel ).
2) Create the chart from the full database. Insert and format the trendline. Make certain that the "hidden and empty cells" setting is set to NOT show data in hidden rows. Depending on where you place the chart, you may also need to make sure that the "move/size with cells" option for the chart area is set to "not move or size with cells."
3) Use the autofilter to show only the subset of the data desired for the trendline. The chart should automatically show only the subset, and the trendline should automatically be based only on the filtered data.

There are, of course, many other possible strategies depending on exactly how you want to interact with the data. I find that the data extraction step is often the most difficult one, so, if this particular strategy doesn't work for you, be sure to explain how you would prefer to extract the desired data for the regression.