Visualize DiskIO as percentages

Hello,

I'm using Kibana 7.

I'd like to show R/W IOPS as a % of the total IOPS and the same thing for R/W MB/s. I started off using the Disk IO (Bytes) [Metricbeat System] ECS Visual Builder visualization as a starting point. I tried using Max -> Derivative -> Math aggregations and used the following expression:

divide(params._all.reads.values, add(params._all.reads.values, params._all.writes.values))

But I'm not getting any data in the chart. Below is a screenshot of the window: