I have attached a pdf file which details the process I follow in excel which leads to the scatter plot data range being just one point rather than the many points I selected.
I will also summarize the process with words in the post-script (P.S.)
I have been able to successfully make data series with many data points, but this problem continues to happen every so often.
To be clear: I am not interested in selecting the data before making the chart. Being able to add data to a completely blank chart seems reasonable and has worked before.ExcelDoesNotKeepSelectedData.pdf
P.S.
With a completely blank scatter plot selected:
Chart Design → Select Data → Add
Left click in Series Y Values box, then left click in cell I6
Hold down Ctrl, then hold down Shift, then press ↓ ( then release Shift and Ctrl ). Note: it is clear that the Series Y values =Sheet1!$I$6:$I:4391
Left click in Series X values box, then left click in Cell K6, then hold down Ctrl, then hold down Shift, then press ↓ ( then release Shift and Ctrl ). Note, it is clear the Series X values =Sheet1!$K$6:$K$4391
Left click OK.
Left click OK. Notice, there is just one point on the graph
With chart still active, Chart Design → Select Data → Edit. Note, the Series X Values and Series Y Values are just Sheet1!$K$6 and Sheet1!$I$6.