Enhance linear charts by displaying different milestones or key points

## Feature Description

Site Kit has many charts that display important information to our users, but it is hard to analyze how my site is doing since my last check a few days ago or after I publish a new post. I think it would be awesome if we can display these certain "milestones" on linear graphs such as "all traffic" widget on the dashboard page and audience overview on Search Console, AdSense, and Analytics module pages.


Here is a jsfiddle example of how to add vertical lines/annotations to linear charts: [http://jsfiddle.net/NC37X/](http://jsfiddle.net/NC37X/).

---------------

_Do not alter or remove anything below. The following sections will be managed by moderators only._

## Acceptance criteria

*

## Implementation Brief

*

### Test Coverage

*

### Visual Regression Changes

*

## QA Brief

*

## Changelog entry

*

Start by inspecting the linear charts in the dashboard's “all traffic” widget and the audience overview charts in the Search Console, AdSense, and Analytics module pages. Determine how milestone events and their dates would be supplied, then verify that vertical annotations appear correctly on each targeted chart without obscuring the existing data.

Written by the indexing model from the issue text.