Visualize data using JSON input in Metrics/Advanced

i want to convert octets to megabytes, why when using JSON input on rollup index it is not working. (Right side in attached picture)
When, using the same for the raw index (not rollup index), it is working: (Left side in attached picture)

Is there a way how to convert values in index which is rollup index?

Well, i found a solution. It is enough if you just changed in Index Patterns the format of needed field from Number to Bytes.

Is it possible to perform some math as well using the JSON input? like divide or multiply 2 fields?

Yes. you can use painless scripting on the advanced section. for example to multiply the resulting value from your visualization you can do,

    "script": { 
        "source": "return _value / 1000"