How do you fill the area under an X-Y scatter chart?

Both fills are possible if you combine the calculate X values in to 1 set of secondary horizontal axis labels.

Because the secondary axis is a date one and dates can only go as low as 1 day. You have values such as 1.3 and 1.4 which would be rounded up.
So the multiplication does the factoring up of your source data. Give the precision of the curve you will need to increase that factor to 1000.