Graph of the code coverage percentage for a specific branch
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=14100)
</details>
<!--IssueSummary end-->
### Problem to solve
Being able to see how code coverage is changing on the `/master` branch is great but some users do not exclusively use that branch. They want to be able to see how code coverage is changing for any long lived branch in their project.
### Intended users
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
<!-- Who will use this feature? If known, include any of the following: types of users (e.g. Developer), personas, or specific company roles (e.g. Release Manager). It's okay to write "Unknown" and fill this field in later.
* [Parker (Product Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#parker-product-manager)
* [Presley (Product Designer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#presley-product-designer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
* [Sam (Security Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sam-security-analyst)
* [Dana (Data Analyst)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#dana-data-analyst)
Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/ -->
### Further details
Reporting feedback from this thread: https://about.gitlab.com/2015/11/22/gitlab-8-2-released/#comment-2376412829
Before the 8.2 release, it was possible to have a tab for a specific branch in the CI page. As the percentage of code coverage was displayed in this page, it was easy to monitor the evolution of the coverage.
There is no longer a tab for a specific branch. So I'd like to be able to select a few branches and to have graphs that show their percentage of code coverage.
cc @ayufan
### Proposal
<!-- How are we going to solve the problem? Try to include the user journey! https://about.gitlab.com/handbook/journeys/#user-journey -->
Extend the graph that was created for master in #33743 to be able to display for any active branch from a drop down.
### Permissions and Security
<!-- What permissions are required to perform the described actions? Are they consistent with the existing permissions as documented for users, groups, and projects as appropriate? Is the proposed behavior consistent between the UI, API, and other access methods (e.g. email replies)?-->
### Documentation
<!-- See the Feature Change Documentation Workflow https://docs.gitlab.com/ee/development/documentation/feature-change-workflow.html
Add all known Documentation Requirements here, per https://docs.gitlab.com/ee/development/documentation/feature-change-workflow.html#documentation-requirements
If this feature requires changing permissions, this document https://docs.gitlab.com/ee/user/permissions.html must be updated accordingly. -->
### Testing
<!-- What risks does this change pose? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing? See the test engineering process for further help: https://about.gitlab.com/handbook/engineering/quality/test-engineering/ -->
* We need to ensure that this is performant, eg; the graph can be drawn in a reasonable amount of time
*
### What does success look like, and how can we measure that?
<!-- Define both the success metrics and acceptance criteria. Note that success metrics indicate the desired business outcomes, while acceptance criteria indicate when the solution is working correctly. If there is no way to measure success, link to an issue that will implement a way to measure this. -->
* Track page views of the graph
* Track changes of the graph
### What is the type of buyer?
The likely buyer for this feature is a Manager placing this capability in Starter/Bronze or above.
### Links / references
issue