When I started developing Charts.CSS I tested all the available methods. There where several use cases where data-* attributes had less flexibility. That is why I decided to use CSS variables.

In your example, the --size calculation is attr("data-value") / attr("data-total"), this is not always the default size.

When you prefer to display stacked chart, you need to change the "total" value. Read the stacked docs, and see the examples on the Simple vs. Percentage section.

In addition, when using CSS variables, you can use math functions everywhere. With data attributes you have to use static value. You can't use calculations like data-total="calc( 2 + 3 + 4 )"