Hi there, I'm sorry I'm not familiar with Diskio, but I can tell you that a field needs to be marked as aggregatable and searchable to be used in a visualization. Here are a couple previous posts in which users solved this problem:
- Fields in Kibana 5 are non-searchable and non-aggregatable
- Kibana 5.2, How to set "searchable" or "aggregatable"
Could you take a look and let me know if they help?
Thanks,
CJ