Update layout md plots - Githubissues

Below is a general list of changes:

Defines plot format, including plot background color, and grid lines and border line color and line width.
Automatically sets y-axis range. User currently does not have the ability to set the y-axis range.
User can define plot title, and group plot lines into a legend group and provide a legend group name.
Added units tests for legend_group and is_visible.

bigladder / dimes