Have you looked into Timelion? It is included in Kibana 5.0 and higher, and will allow you to do the calculation you are trying to do. Then Timelion graphs are able to be added to your Kibana dashboard just like Visualizations.
Here I made a simple example. I am taking the average bytes over time and adding the average machine.ram (divided by 1000000 for simlar data) over time.
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