The mappings are held in Elasticsearch, Kibana uses those mappings to control what it's able to do with the fields.
Check the mapping for your index by running curl -XGET "http://hostname:port/logstash-*/_mapping. If the field is not mapped as a float here, something is going wrong with your logstash.conf.
If your using a mutate filter to do the conversion, this reference could be of help.