IOX: Ability to visualize multiple fields & field names reflected in axis drop down selection

**Proposal:**
Add the ability to visualize multiple fields simultaneously.

**Current behavior:**
You can only visualize one "_value". It's unclear which value is being visualized.

**Desired behavior:**
It would be great if you could select multiple fields for the y-axis drop down.
It would also be useful if instead of "_value" the dropdown included the columns that you wanted to visualize. I don't know if it's intuitive that "co" is being visualized here.

**Use case:**
I believe the ability to graph multiple fields was available with 1.x and influxql. The requirement of using multiple yields was a big hurdle for Flux users who were accustomed to being able to visualize multiple fields with InfluxQL. I believe SQL users expect to be able to visualize multiple fields simultaneously as well.

Thank you!