April, per your description I think you're looking more for reference lines than trend lines. Something like this:
If so, then all you need to do is change the Year pill on the column shelf from 'discrete' (blue) to 'continuous' (green) and the reference line option will be available to you. Then select 'constant' and pick a year. I would say repeat as often as you like, but I've never tried putting a whole bunch of them on a viz so I don't know if there is a limit (anyone?).
Hope this helps, please post back when it's done, I'm interested in the results of your viz.
April, you're welcome! But don't forget the deal, you have to post the link back here when you're finished, right? I really am curious about the topic and what legislative measures change the dynamic. Thanks,