Number of HN submissions over 3 years: http://i.imgur.com/r9Ayvb1.png
Number of HN comments over 3 years: http://i.imgur.com/4FwglA8.png
Number of HN submissions over 3 years: http://i.imgur.com/r9Ayvb1.png
Number of HN comments over 3 years: http://i.imgur.com/4FwglA8.png
Of course, this scenario assumes that # of submissions follows a power law distribution: a small percentage of posters make the majority of submissions. That pattern emerges on many content sites, especially sites with active forms of communal self-moderation. I have no idea if it's the case here, but it seems feasible.
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.
If the number of comments, on the other hand, hadn't gone up since 2012, I'd be shocked.
Here is a chart of the comments: http://i.imgur.com/4FwglA8.png