Hi Anitha,
Its easy and simple, follow below steps:
1. Create a bar chart as usual.
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2. Add Grand totals from analytics pane.
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3. Now click on the grand total bar and click on automatic.
4. Select average in the drop down.
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5. Now a average column for the chart of all values is shown at the last column.
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6. Now edit the alias name for the column/bar to average or grand avg.
Hope this helps you.