x-axis datetime formats

__Proposal:__
`yyyy-MM-dd`, `dd MMM`, or even allow a format string to be entered.

__Current behavior:__
The formats are a hard coded list and they all include the time.

__Desired behavior:__
An x-axis label option for datetime values includes format strings that include only the date and not also the time.

__Alternatives considered:__
Describe other solutions or features you considered.

__Use case:__
The time is meaningless when data is grouped daily, weekly, or more coarsely.

The issue does not name files, tests, or an entry point. Start by locating the x-axis datetime formatting UI; done when date-only formats such as yyyy-MM-dd or dd MMM can be selected for coarsely grouped data without displaying the time.

Written by the indexing model from the issue text.