Hi Anitha,
To add average line already aggregated fields in a chart, you can use fixed LOD to show the overall average over each average.
Follow the below steps:
1. Construct Avg bar chart for requiring dimension and measure.
2. Add a Fixed LOD expression using calculated field as { AVG([MEASURE]) }.
3. Now add the created field in details shelf.
4. Add an average line on the bar chart and edit the line.
5. Once the edit dialog box opens, click on value drop down and select the created field from the list.
6. Now the average line is shown in the average bar chart.
Hope this helps you.