Your question is mainly about labels. However, you also mentioned the possibility of splitting an element into several entities.
So you have a stacked bar – like a graph that raises heavy loads.
1. Consider a table of comparative values and a sorting and exploration capability.
There are some trade-offs (like perhaps more reading than a visual gestalt), but this helps to advance the data in a sortable manner and can show a small percentage of categories without eye strain or interpretation.
Summary tables can:
- Show totals (the summary at the bottom so that users know the total, if it's important)
- Show parts of everything: column% allows users to see relative values
- Split category payments (instead of vertical lines, the user should see the small volume categories, simply label the category and allow the rollover details)
- Sort for a quick comparison
- Scale: As new categories are added, you can add an extra row
- Show Labels: Your current labels always force the user to find which color matches which category
Is your visualization part of a larger dashboard?
If this is the case, you can place color captions in the table itself, to link them to other nearby visualizations:
2. Try a horizontal bar graph instead
In the example of the stacked area, users must use the height to see the magnitude comparison at a glance.
With a horizontal graph, you can take advantage of the ability of humans to evaluate values by length.
Show Me the Numbers, by Stephen Few (94):
Visual perception is highly suitable for detecting differences in lengths of objects sharing a common baseline, but not sufficiently adapted to distinguish differences between 2D areas.
I do not know if I really understand your use case, but it seems that the analysis of categories, their magnitude, the labels and the need to indicate split payments could justify the. experimenting with a different type of visualization.