Add specific fields into the timechart OTHER category.

I'm generating a report of the daily usage of my users indexes over the past week using this search:

earliest=-7d@d latest=@d index="_internal" source="*metrics.log" per_index_thruput 
| eval GB=kb/(1024*1024) 
| bucket _time span=1d 
| convert ctime(_time) as timestamp 
| timechart span=1d sum(GB) by series

This works well, except the "_fishbucket" shows as one of the values charted.

I would like to combine"_fishbucket" and a few other fields into the "OTHER" category, but the only methods I can think of appear to drop them completely from the report.

Any suggestions?