IMO, the quality has degraded due to more people viewing the site, not the submissions. If I get my post on the frontpage of HN I can expect roughly 1,000 hits per hour. This wasn't the case 4 years ago when I first starting lurking here.

There are two problems with this:

1. More people (of poor quality) upvoting stories that attract the most attention (link bait, stories that aren't relevant, etc)

2. More people (of poor quality) upvoting comments that are not relevant to the discussion.

No graph will be able to chart this - it's purely subjective.