Regresson Analysis for Net promoter Score (NPS)

#1
Problem: I am trying to calculate with Excel a regression Analysis for NPS scores but outcome, I am sure is not right. Not sure I am using the right Variable.

Background: The Net Promoter Score(NPS) is an index ranging from -100 to 100 that measures the willingness of customers to recommend a company's products or services to others. “On a scale of 0 to 10, how likely are you to recommend this company’s product or service to a friend or a colleague?” Based on their rating, customers are then classified in 3 categories: detractors, passives and promoters, and NPS is calculated.

What I am doing: Using the scale of 0 to 10 as a dependent variable. Using time a service lasts (In days). When I use Excel scatter plot to see the dispersion of the dots, basically, it represents 10 lines of dots from Y axis. (I think I am doing something wrong):

Any idea what would be happening? Am I using the wrong dependent variable? Should I use the score between 0 to 10 or the NPS score that is calculated with it?
Thanks!
 

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#3
Cos the correlation that I get is too low and I am not sure I am picking the right dependent variable (Score from 1 to 10).

Also, my sample is about 3000 customers, with this decision time going from 0 days to 400 days, and when I group the decision time, calculate the NPS score for each group and plot it, mainly, as greater is this decision time, as smaller is the NPS.