Line and Stacked Column Chart but with changing Line Y-axis

Hello 
I am looking to create a line and stacked column chart but with changing line y-axis. To be more precise, I want the line to be constant at 100, but when todays date is later than XYZ then it needs to jump down to 80 forming a stair case like shape. I do not want the whole line to jump down to 80, but the line from XYZ date 
My graph looks like this, and I drew waht I need to be created. 

Please contact me if you have questions.

Best regards,

Rasmus