How can I avoid using a custom column to check if a field is null?

Right now, I use the following custom column:

case(isnull([TimeStarted]), 0, 1)

with this summarization

(in this summarization, WasStarted is the custom column checking whether TimeStarted was NULL).

However, this breaks my visualizations. Specifically, now when I click on an area in a chart it shows "show records" instead of "show (my model name)".

Can I avoid using a custom column here?