How to create math expressions?

[isarns]

I'm trying to create this simple expression to be used as a target in my panel.

{
  "datasource": {
    "name": "Expression",
    "type": "__expr__",
    "uid": "__expr__"
  },
  "expression": "1-($B/$A)",
  "hide": false,
  "refId": "C",
  "type": "math"
}

How can I create one of those? Can you point me to the docs?

[Duologic]

This isn't supported by Grafonnet today.