Excel graph pulling in wrong data point for one date

A picture isn't much use.

Please attach the workbook.

Unfortunately the attachment icon doesn't work at the moment. So, to attach an Excel file you have to do the following: Just before posting, scroll down to Go Advanced and then scroll down to Manage Attachments. Now follow the instructions at the top of that screen.

I'm not sure I understand. The chart is merely plotting all the points in rows 27 to 107. In row 28 is a point for 28 Sept 2018 that is, as you note, 20.05%. In row 100 is a point for 30 Sept 2018 that is 0.5%. Since both points are present in the source data, the chart correctly displays both points. If you don't expect the 30 Sept 2018, 0.5% point to be present in the source data, that suggests something is wrong in the source data. It does seem odd that the source data would have a 2018 point down in the middle of a bunch of 2012 points, but I don't have any way of knowing the history of this data (where it came from or what may have been changed after importing to Excel). My guess is that something is wrong with that date in A100. Debugging means going through the importation and editing process that leads up to this point until you know why that point became 30 Sept 2018.

If it is as simple as declaring a bad data point, I will add an additional observation. I don't know what error checking options you have turned on (I have most of them turned on), and Excel gives me a little green triangle error flag for C99. The "error" is "inconsistent formula", and C99 does seem to have a different formula from the rest of the column. I don't know if the formula in C99 is correct or not, but it is adjacent to the erroneous data point, so something else may have happened in the history of these rows to change more than the value of A100.