keep series on drill down

Hi, have a line graph of a measure that shows % items ok per room (x axis month). At start I have the total percentage per month and I can drill down to look to the percentage in each of the rooms. That works great!

Now my issue, I want that each room to compare itself to the global % (value before dill down), so when I drill down I'll like to keep the value of the total, or draw a line with the total %.

Any idea how can achieve this?

Thank you!