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...