Customization of Legend

### Background
See #342

Rendering a static legend will require allowing the user to customize what shows up in the legend.

This could be a set of options that applies to both the floating legend and static legend, or more fine grained and apply to each independently.

There are two main options here:

- Controlling which columns should be excluded from the legend(s)
- Ability to rename (provide an alias?) for items in the legend
- For static legend this would be control over the string that appears next to each colored dot
- For floating legend this would be control over naming of each column

These options do not apply to all visualization types

Start by reading issue #342 and identifying the legend-related entry points for static and floating legends. Clarify the option API, which visualization types support it, and how excluded columns and aliases should behave. The work is done when both legend modes expose the agreed customization and the supported visualization types are documented or tested.

Written by the indexing model from the issue text.