I am using the FIS API, and it seems that the BINS parameter (that define the size of histogram) is limited to 6, whatever value I put (sometime I have less values, but never more).
Can you send an example of the FIS request here, masking the part of the instance ID, so that we can take a look?
Generally though if your evalscript has only 6 discrete output values, then FIS will be limited to 6…