This should fix #243 and #94. As part of this, I also draw labels in a separate loop, so it supersedes #161.

Thanks for the suggestion, @stockiNail. I hadn't considered using a drawTime property at the label node. I do like that better, although I have a question about how it should be designed: Should setting a chart-wide default drawTime for labels be permitted? That would mean that the chart's options.plugins.annotation has a label sub-object with only one usable property. (The line annotation plugin's labels implementation doesn't otherwise pull any label defaults from the chart's options.plugins.annotation - maybe that's worth raising separately, if it's an issue for anyone. Or maybe there's a way to better merge in defaults automatically; I'm still learning my way around the Chart.js APIs.)

I noticed that text annotations were listed under "To-do Items" (and it's probably something that our project will eventually need). I'm guessing that a hypothetical text annotations feature would use the same label sub-object as lines (to keep a consistent interface) and so whatever we decide here would work for that, too.

Should setting a chart-wide default drawTime for labels be permitted? That would mean that the chart's options.plugins.annotation has a label sub-object with only one usable property. (The line annotation plugin's labels implementation doesn't otherwise pull any label defaults from the chart's options.plugins.annotation - maybe that's worth raising separately, if it's an issue for anyone. Or maybe there's a way to better merge in defaults automatically; I'm still learning my way around the Chart.js APIs.)

@joshkel you would like to set the label drawTime default into annotation plugin options in order that all labels of all annotation can inherit from that. In this case the labelDrawTime should be set, agree with you. Do you think this is the good use case?
In this case, forgive because it makes sense to me as well, I didn't think about that case even if usually this decoupled drawing should be an exception, I guess, and then could be configurable for a single annotation.

I noticed that text annotations were listed under "To-do Items" (and it's probably something that our project will eventually need). I'm guessing that a hypothetical text annotations feature would use the same label sub-object as lines (to keep a consistent interface) and so whatever we decide here would work for that, too.

I was thinking about that (also to have label on the box and all other annotation types) and maybe an abstract label class is needed and every annotation could extend and implement special labeling.
This is valid (in my opinion) also for TextAnnotation (not implemented yet).
But I didn't do yet because I'm working on other stuff, sorry.

@joshkel apologize, I forgot an important thing on my previous answer.
@kurkle taught me that the annotations are now Chart.js elements and therefore you can configure the annotation (by its type) also in Chart.defaults.elements and/or chart.options.elements, like point, bar, arc or line (default elements of Chart.js).

Therefore if you want to configure all line annotations (but this is valid for all annotation types) with a specific drawTime (and then let's assume to use drawTime for label as well), you can do something like that:

options: {
   elements: {
       lineAnnotation: {
         drawTime: 'beforeDatasetsDraw',
         label:  {
            drawTime: 'afterDraw'
         },
   }
   ....
   plugins: {
      annotation: {
         annotations: {
            myHorizontalLine: {
               type: 'line',
               scaleID: 'y',
               value: -90,
               label: {
                  position: 'center',
                  font: {
                     size: 16,
                  },
                  backgroundColor: 'red',
                  content: 'This is a label',
                  enabled: true
               }
            },
         }
      }
   }
   ...
}

You can do the same into Chart.defaults as well.

In this way you can configure the drawTime default option for all labels of all line annotations.

Unfortunately a piece of code is currently missing into plugin but I think it makes sense to add:

https://github.com/pepstock-org/chartjs-plugin-annotation/blob/a46915f3e409928281f1e0c9d550bc524c1bddc3/src/annotation.js#L136

el.options = merge(Object.create(null), [elType.defaults, chart.options.elements[elType.id], annotation]);

This line is of my branch where the options are calculated differently for your code (when merged and after conflicts resolving, it should work changing that).

In this way you can get the drawTime also into your label as default for all line annotation and then the drawing can use drawTime (instead of labelDrawTime) for the label as well.

If you agree on that (to use drawLine for label and use the elements for default configuration), I can add the "merge" update to my another branch and submit another PR.

I think I like the label.drawTime more. Lets go with that.

I'm planning to provide some utilities for this in the core, no need to write the resolution path in every plugin (and extension):

el.options = merge(Object.create(null), [elType.defaults, chart.options.elements[elType.id], annotation]);

I'm planning to provide some utilities for this in the core, no need to write the resolution path in every plugin (and extension):

Sounds good!

Apologies for the delayed reply.

I changed to label.drawTime, as suggested, since that definitely seems like a cleaner design.

Do you want me to do anything with merging options now, or wait until other PRs and/or core work?