Excel graph pulling in wrong data point for one date

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.