In kibana how to draw a graph which should give me average time given with 2 dates(start date & end date)

I am having 2 date field one is PR open date other isPR closed date in my ES _source with PR num, project name, etc.
Now In kibana I want to draw a graph which should give me average time to close a PR.
how to achieve this?
one more doubt how to do mathematical operation on 2 fields of same data type for ex: addition of filed1 + field 2 or subtraction of field1 - field 2

Hello @Amarsha_N.G - Have you tried using scripted fields? https://www.elastic.co/guide/en/kibana/current/scripted-fields.html