Evaluates the fill aesthetic mapping in the context of the plot data
to extract fill levels. Handles both simple column references (e.g., fill = cyl)
and expressions (e.g., fill = factor(cyl)).
Evaluates the fill aesthetic mapping in the context of the plot data
to extract fill levels. Handles both simple column references (e.g., fill = cyl)
and expressions (e.g., fill = factor(cyl)).