What are the steps to reproduce your problem?
1. Create a report View Process Instance Count
2. Group by Variable
3. Choose a Date Variable
4. Visualize as Bar or Line Chart

What is the problem?
Sorting is wrong: Left is latest date, right is oldest date

What would be the expected behavior:
Sorting should be the other way round - same as for group by start & end date

Hints (optional):
Sorting works fine in tables.