How to exclude null values in running totals table calculation

[Nadirfb]

Hi, 

I want to create a graph with cummulative values (running calculations):

running_total(${X.amount})

The problem is that if my date filter is one complete year and some months:

2019/01/01 - 2020/08/08

, the running calculation is working properly for the 2019 but for 2020 is showing values for every month not just up to August. So is summing null to the previous month for the last months giving the value of august from august to december(horizontal line in the graph).

I would like to see the complete area for 2019 but a line from january to August in 2020

Any help/ideas?