Add annotations (#1)

* Add support for annotations

Annotations are lines and boxes that show up on
line charts with time axis. They can be used to mark
events and periods.

* Refactor annotations fetching

* Add annotations presence tests

* Add README for annotations

* Adjust charts annotation ui

* Add an ability to override colors

* Allow overriding annotation service

* Clean up rebase

README.md

@@ -266,6 +266,32 @@ smart_columns:
   status: {0: "Active", 1: "Archived"}
 ```
 
+### Annotations
+
+Shows overlay lines or box ranges for line queries.
+
+Suppose your sales data and your deployments data, given a query:
+
+```sql
+SELECT date_trunc('hour', created_at), sum(value) FROM sales GROUP BY 1
+```
+
+You might want to see the influence of a deployment for those sales.
+
+```yml
+annotations:
+  deployments: SELECT date, name FROM deployments WHERE date BETWEEN {min_date} AND {max_date}
+```
+
+You can also show periods:
+
+```yml
+annotations:
+  holidays: SELECT min_date, max_date, name FROM holidays WHERE (min_date, max_date) OVERLAPS ({min_date}, {max_date})
+```
+
+Conditions for those queries are optional, but they will help to only fetch the relevant annotations for a particular chart.
+
 ### Caching
 
 Blazer can automatically cache results to improve speed. It can cache slow queries:

app/assets/javascripts/blazer/application.js

@@ -12,6 +12,7 @@
 //= require ./chartjs-adapter-date-fns.bundle
 //= require ./chartkick
 //= require ./mapkick.bundle
+//= require ./chartjs-plugin-annotation.js
 //= require ./ace
 //= require ./Sortable
 //= require ./bootstrap