You'll want to check out reference lines and bands. Here's the online help: http://onlinehelp.tableausoftware.com/v7.0/public/online/en-us/reference_lines.html
The short of it is, if you right click the axis, select "Add reference line", then select "Band" you can hard code a constant range from 0 to 75% and then repeat the process to add another reference line (band) for 75% to 200%.
Hope that helps!
Oh, and welcome to the forums! Glad to have you!