Add temporal line chart annotations

[khasinski]

I'd like to propose an annotations mechanism which allows overlaying data on top of the line charts with a temporal axis.

The main use case is to keep a log of events (whether they are points in time or ranges) and to see the effect of those events on multiple metrics we track in Blazer. A good example would be to track deployments, a marketing campaign duration or holidays and overlay them on top of revenue or visits data.

Here is an example:

blazer.yml

    annotations:
      deployments:
        SELECT created_at, name FROM deployments # datetime + string, results in a vertical line + label
      holidays:
        SELECT min_date, max_date, name FROM holidays # two dates + string, results in a colored range box

Result

Developed together with @twratajczak

Fixes #403

[khasinski]

I've added the ability to override colors, even though it requires extending Blazer::Annotations and setting it with Blazer.annotations. This might be used to easily mark for example weekends with a light gray. Helpful if the data has a cyclical nature.

Here is an example of that (to be put in a rails initializer)

class CustomAnnotations < Blazer::Annotations
  def call(result)
    min, max = result.rows.map(&:first).minmax
    super + weekends(min, max)
  end

  private

  def weekends(min, max)
    (min.to_date..max.to_date).select(&:on_weekend?).each_slice(2).map do |weekend|
      {
        min_date: weekend[0],
        max_date: weekend[1].end_of_day,
        label: "", # Empty labels are not rendered
        color: "#FAFAFA",
      }
    end
  end
end

Blazer.annotations = CustomAnnotations

Results in (notice that it works with existing annotations):