Supposedly the March algorithm update was about query relevance [1], and maybe your site just isn't the most relevant for a very general search like "polygraph". My site used to be on page 2 for "Photoshop Plugins", but after the March update it crashed to page 5 (according to Search Console). Since my site isn't about every Photoshop plugin, just the ones I make myself, perhaps that's fair.
I'd definitely recommend playing with the new Google Search Console Beta - you can do a search on the specific "polygraph" keyword, and it will show you what your site's Google rankings were for that keyword over the last 16 months. Click on Performance in the left menu, set Date to Last 16 Months, then add +New -> Queries -> Query Is Exactly -> polygraph. My guess is you'll see a big drop on March 7 / 9. Also worth checking to see what your Click-Through-Rate is.
[1] https://www.mariehaynes.com/march-9-2018-google-algorithm-up...
If I interpret things correctly, the Google Search Console Beta suggests that the situation is worse than I imagined. I have taken a screen shot that you can view here:
https://antipolygraph.org/graphics/google-search-console-ap-...
As you'll note, over the past 16 months, Google reports 147,000 impressions for the search word "polygraph," with an average position of 8.4, but only 381 total clicks. And the number of impressions and clicks flatlined somewhere around March. What do you make of this?
If you now click where it says Average Position, you'll get a purple chart of your average position/ranking for that specific keyword over time. (You can actually click on each of those boxes to turn the graphs on & off.) In my case, some of my keywords have a very obvious & sharp drop in ranking beginning around March 7 / 9. My guess is you'll see something similar.
If I'm right about the March Google update, then there isn't anything wrong with your site. It's just that people aren't clicking on your site when they search for "polygraph" (0.3% CTR is rather low, as you can see compared to your other keywords) or Google feels your page isn't the most relevant to their query. You could try making a page on your site focused specifically on that keyword, but you'd be fighting against Wikipedia & Merriam-Webster for the niche.