Select from all columns for heat map

**Proposal:**
Be able to select for columns with string values for the y-axis for the heat map.

**Current behavior:**
You can only select for long or timestamp.

**Desired behavior:**
The ability to select other columns of type string or int.

**Use case:**
Users want to be able to create heatmaps that measure the magnitude of values based off of tags.

Community user question:
https://community.influxdata.com/t/cant-use-heatmap-with-categorical-variable-as-an-axis/25314/3