Please note that this is a logarithmic scale; every vertical division to the right makes the word is ten times more common, two divisions make it 100 times more common, etc.
If you want to see a standalone image at full resolution, it’s hosted here.
The diagonal black line shows the overall trend: the Scripps National Spelling Bee winning words have become more and more unusual as the years pass.
As noted above, this is a logarithmic scale, and thus there cannot be a zero value. (It’s a math thing.) Words without bars at all did not appear at all in the Google Ngram Viewer for that year. According to Google, this means the word appeared fewer than 40 times in their corpus for that year.
The only word in this list that does not appear in the Google Ngram Viewer for any year is esquamulose, the winning word of 1962. It is an adjective meaning “not covered in scales”.
The list of winning words was from Wikipedia; for more information about the Scripps National Spelling Bee, see the Wikipedia page or their home page.
EDIT: There was a discussion on Reddit’s Data Is Beautiful forum about this graph.
No smoothing was used in the Google Ngram Viewer, in order to get the most accurate results. The Google Ngram Viewer only goes to 2008 at present, which is why the graph stops there; the winning words since then were Laodicean, stromuhr, cymotrichous, guetapens and knaidel.