A quick way to accomplish this is to manually set your Y Axis min and max values based upon your data (assuming 0-100%). Then set your major unit to 0.8 (80%), and your minor unit to 0.1 (10%, or whatever works). Then use the Major Gridline to act as your 80% goal line.
![Pivot Chart Example](../../I/static/images/2f66a54b5238634c0fe7488040c938b8c5ef2e5131364692906582c7b4151206.png)
The downside is that you can't label your other minor gridlines (directly as a formating option). However, if your chart's message is about progress in relation to the goal, this should suffice.
EDIT: I'm not sure why the 30% line didn't show up here, it's in the original, but went missing when I created the .png, so it's not an Excel issue and shouldn't be a problem for the solution.
IMO, stay away from Pivot Charts-they impose to much MS formating. Consider a non-Pivot chart based upon your pivot table data and then you have much more flexibility. – dav – 2013-03-18T15:04:49.423