How to sorting Pie chart by custom order

I'm creating age backet chart. I want to sort pie chart by custom order.

Now, default pie chart is displayed by

1. 36 to 40

2. 41 to 45

3. 31 to 35 -----

10. Under 20

It looks the chart is ordered by volume of each element.

I want to sort with

1. Under 20

2. 20 to 25

3. 26 to 30

4. 31 to 35 -----

10. Over 60

Can I change the order?