The data illustrate a steady decrease in payroll-point efficiency. The more a club pays its players the less efficient is that spending. The reason for this is that salaries increase non-linearly and points increase linearly.
The graph below shows that, in general, the core Soccernomics conclusion that more payroll means more points can be seen in these data as well. But there is a lot of spread in the data too, suggesting that payroll is not the entire story.