Add chart functions "x()" and "constant(3)"
- This might be useful to draw a line like "goal" or "profitability".
- This might be useful to pull data from an external source.
- For composable functions like "add" or "subtract", which are useful in manipulating ETL datasets, these value functions will make testing easier.
Added a constant(256) function:
Reviewed By: amckinley
Maniphest Tasks: T13279
Differential Revision: https://secure.phabricator.com/D20443