There are diferent ways to realize this objective:
1) use dimension limit (is the third tab of properties of chart) and set the percentage you need
2) if you have only one expression you can write: if(opercentage >= 20%, percentage, 0) and in the presentation tab set
"suppress zero values"
3) in the dimension you can aggr the expression and test the result setting other items to null, then exclude null values
Let me know