Overlay timeline with offset time

Hello - I'm trying to work out how to apply an offset to the time and overlay that onto the normal time.

E.g., I have a forecast that tells me I have 100 customers arriving at 1230, shown on a line graph with timeline along the X axis. I want to overlay that with a line for Staff that are needed at an offset of 20 minutes after that. Therefor if the customers to staff ratio is 100/15, then I should see a matching line at 1250 showing 15 staff. 

Any thoughts on how to configure this? Thanks!