Two annual tables can use the same category label while counting different things. Before comparing the numbers, read the definitions attached to each edition. A stable heading is useful, but it is not evidence that the underlying rule stayed stable.
Construct a small example
Imagine a club originally labels members under 18 as “junior” and later changes the boundary to under 16. A fall in the junior count could partly reflect the new definition. This is an illustrative example, not an observation from a real dataset.
Write the rule beside each year and identify which people would move category solely because of the change. That makes the comparability problem visible before any percentage calculation.
Look for a documented bridge
The publisher may provide revised historical figures or a note explaining the break. Use that information if available and record which version you used. Do not silently combine incompatible categories because the chart looks cleaner that way.
If no bridge exists, present the limitation clearly or narrow the comparison to a consistent period. The task is not always to produce one continuous line. Sometimes the most accurate result is a table that shows where a definition changed and why the values on either side need different treatment.