Add UTC versions of TimeUnit

[jeffesp]

This isn't the prettiest change, but it seems to work in my local testing.

I'm unable to generate accurate graphs without it.

[jwoLondon]

Thanks very much for suggesting this. I had meant to add UTC time units in an earlier release so your prompt has been helpful.

Rather than add a set of another 24 types to what is already rather a cumbersome set of time unit types, I would prefer to add a single Utc tag that takes any TimeUnit as a tag. For example,

 encoding
        << position X [ PName "date", PmType Temporal, PTimeUnit (Utc YearMonthDateHours) ] 

I've created a utc branch on the repo for you to have a look at and test. Would that work for you? If so, I can merge it into the master. Unfortunately, because we are forced to add to the TimeUnit type, this will bump the release as a major change, so ideally I'd like to wait to merge until other changes have been incorporated - specifically to have some Vega support in addition to Vega-Lite.

[kachkaev]

Hi @jeffesp, thanks for this contribution!

I see you've started to make a fork with your functionality that contains a new url. etc. Could you please do it in a new branch (e.g. develop) and keep your master at 14f9ccc for now? This will make this PR mergable if @jwoLondon decides to 😉 It'd be better if there was an option to change PR's branch name after its creation so that you could use your master, but I guess GitHub does not support that.

[kachkaev]

Oops – commented simultaneously with @jwoLondon 😅

@jeffesp if you're fine with the new functionality in a different branch, no need to do anything – we'll simply close this PR later on.

[jeffesp]

@jwoLondon @kachkaev

The changes in the utc branch look great. I agree that the extra types were very cumbersome. Thanks for the prompt reply here and feel free to drop this merge request whenever.

[jwoLondon]

OK, that's good to know - thanks. If you want to use the utc branch locally for now I don't have any plans for further changes to the API for dates in the next release, which realistically won't be for a few weeks yet as we work on other additions.

[jwoLondon]

@jeffesp The latest release of elm-vega (2.2.0) now includes UTC support. It uses a function utc to convert any TimeUnit to its UTC equivalent, rather than a new type constructor. This means it can be released without a major version bump.