Compare line page

[nmqng]

Description

The purpose of this pull request is to add a comparison line graphic page based on the design doc.

Fixes #953

Type of change

• [ ] Note merging this changes the database configuration.
• [x] This change requires a documentation update

Checklist

• [x] I have followed the OED pull request ideas
• [x] I have removed text in ( ) from the issue request
• [x] You acknowledge that every person contributing to this work has signed the OED Contributing License Agreement and each author is listed in the Description section.

Limitations

Currently, when a shifting to/from a leap year from/to a non-leap year, a warning show up to tell the user that the original graph and the shifted graph will not align appropriately. However, in the future, we would want to find a way that can make the two graphs align.