Line chart reference Lines

[JoshSchoen]

*I'm submitting a to add options for reference lines in line charts. (check one with "x")

[ ] bug report => search github for a similar issue or PR before submitting
[X] feature request
[ ] support request => Please do not submit support request here

Current behavior
You are not able to add reference lines to the line charts.

Expected behavior
Add up to 3 reference lines and range and labels to line charts. This is needed to indicate min, max, averages and show confidence levels.

What is the motivation / use case for changing the behavior?
Ability to add up to 3 reference lines and range and labels to line charts.

There is an open PR #422

Please let me know if there is anything that needs to be done in order to move the PR forward.

[marjan-georgiev]

Merged into master.