The reason why you see that null one(s) is because you select the whole column(s) as data source (ex. A:C) instead of only select the range with data (ex. A1: C20). You have 2 options:

1. by selecting A:C, you have to add a filter in chart setup section to exclude those rows without data
2. change the data source range to A1:C20 The 1st is better as it'll be able to handle the potential change of the data.

The reason why you see that null one(s) is because you select the whole column(s) as data source (ex. A:C) instead of only selecting the range with data (ex. A1: C20). You have 2 options:

1. by selecting A:C, you have to add a filter in chart setup section to exclude those rows without data
2. change the data source range to A1:C20 The 1st is better as it'll be able to handle the potential change of the data.