Dynamic x axis and legend

Hi All,

I used this forum to help me create a dynamic area chart (thank you again to Sktneer for solving my initial problem), I am not trying to make the x axis dynamic, such that the months which do no have data in are not shown.
I am trying to avoid removing the months which do not currently have data, because the dates represent a fixed contract length.
I've attached an example file to further illustrate my point.
I am also trying to adapt the legend on the chart so that only the grades that are currently selected show on the graph?

Any/all advice would be welcome.
Thanks in advance.

Many thanks for this Sktneer, I am trying to achieve this with a checkbox chart rather than with option boxes, will it work in the same way?

Thanks for all of your help Sktneer, much appreciated.