* Added comments to properties that don’t have any.
* Added comment to `get_chart_widgets`, `customers_vs_guests` since it
was `method function`
* Added missing `exit if accessed directly`
* Updated `get_main_chart` comment since it is wrong.
* fixed indentation - swapped spaces for tabs at start of line, swapped
tabs for spaces min-line for alignment
* removed instances of `@return void` and `@access public`
* improved readability
@see http://make.wordpress.org/core/handbook/coding-standards/php/
Aristeides Stathopoulos
Fredrik Forsmo
Claudio Sanches
George Stephanis
Mike Jolley
Tamara Zuk
Coen Jacobs