In my XY Charts, I set the series according to the columns I want to view.
The Gant chart seems similar to a horizontal stacked bar graph.
Data sources
It looks possible to use one data source or separate data sources
It looks like either source technique can fill the 0s from the query or scripts.
Query speed
Your current technique seems to have minimized data in the query.
I think a single source query with zeros inside it would be the fastest.
I don't see data source specifics, but I learned most the stacked bar chart here:
Stacked Bar Chart Detailed Steps