Not sure what you mean for filters to specify the Slices of the pie but assuming you want to aggregate some index by a field and get the count of documents per field value and then filter out some values you want to hide, I think you can use the filterrows function in Canvas or maybe simply put a WHERE condition in your SQL clause.
In this screenshot I get from the flights sample data set a count of documents per Carrier and then in the expression editor I added a filterrows to only keep those for JetBeats and ES-Air
But as I was mentioning, you can do the same from the source just adding a WHERE clause
Does this help?