Area fill between two line graphs in well log viewer

Child of #1020
As a user I want the ability to able to Area fill the section intersected by 2 lines, so that I can get valuable visual feedback by looking at the widget.

Acceptance Criteria:
1. In the case where the user has added 2 lines he should be asked to select the Area fill with reference to which line. (Line 1 or Line 2)
2. Once the user has selected the reference line he should then be able to select the Area fill for both (Left and Right)
3. In the case where the user has added 2 lines, the user would always have to give 2 colors with reference to the line.
4. In case if the user has selected a single line, then in that case he should be able to choose only 1 color for area fill which would be either left aligned or right aligned.

Refer the image below to visualize above. Or contact Abhishek Pal or Chauhan or Jay

https://drive.google.com/open?id=0B6Z6tlkIQkdjOGZoZXRQazZxVXB4V2tsVXVhQjlIczJnSmI0