This class has been created to hold the graph options for TranslationStats.

This was introduced in rETRAc0cda5a1b4b8ebfdd1a6698e8f3d6f083dc898f1 (T134634: Replace phplot graphs with JavaScript library solution)

Following improvements

1. Remove dependency on FormOptions
2. Add getters for all the values
3. Perform validations and throw exceptions for invalid values instead of converting and truncating the values.

Also see: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Translate/+/622548/10#message-80212ebdf1a4e0879cc0ba9f18a56bc54bdaac3b