You can make the reference line dynamic using a parameter.
Follow the below steps to make a dynamic reference line -
1. Create a bar chart with the region and average sales.
2. Add a parameter for the selection.
3. Then create a calculated field to match the parameter and calculate the aggregate that you want to use as a reference line.
Example, IF [Category] = [Cat Parameter] THEN [Sales]
4. Add the created calculated field in Details shelf.
5. Now Add an Average reference line on the chart click on the edit line and select the calculated field.
6. Click OK.
Hope it helps you.