First welcome to tableau
Kinda hard providing a solution without seeing the TWBX workbook
in general and from your viz - you have "bined" the data - typically when the is a void in the data - Tableau will not plot those unless told to
see the path below
also the Null values indicates there are blanks in the data - can be fixed using different ways or just hidden/filtered out
to get the most out of your post suggest you review the Required Readings on the site Getting Started in the Forums
If this posts assists in resolving the question, please mark it helpful or as the 'correct answer' if it resolves the question. This will help other users find the same answer/resolution. Thank you.
Can you, please, share your workbook, so it will be easier to give clear answers.
First think I'd suspect - NPS(bin) (calculated field) need some customization to use 1 as size of bin.
Let me know