Colored regions based on layout calculations

[Miss Cognos]

Hi,

I want to insert a colored region in my chart.
I'm using a normal line chart with the quantity on the y-axis. The x-axis displays a date and additionally order numbers.

I now want to add a colored region for the last Order on the x-axis. However I don't know what to insert into the layout calculation to achieve this goal.

I tried the "category index" and the "percent on Axis" positioning but those are not dynamic. Layout calculation seems to be the right choice for dynamic positioning however I'm not able to insert a calculation which works.

Any help would be highly appreciated