Hi @cheriemilk!
I will try to help here 
On a pie chart, top hits aggregation only works with numbers. There is a previous Discuss post around a similar issue.
To use it you have to first create buckets using split slices, and than use the top hit aggregation, that aggregate the top hit results, sorted by the second field on the editor, on each specific bucket with the specified aggregation function (sum, min, max, avg).