Instruction:
1) Data Validation is a good function to create a list of selection. You can use Data Validation to create a list of month as picture below.
2) Vlookup or Hlookup is a good function to obtain data from a table. After selecting a month, use Vlookup to obtain data from table below. Thats all. After select a month, the data from selected month will show at table above. Then, create pie chart using table above.
