I'd be interested in hearing your take on this. My unscientific conclusions include the following:
1. Henry Ford is the dominant presence among American innovators, and has been for the last century. He died in 1947, which may explain the spike in interest in the 1950s as the press and historians tried to evaluate his contributions. Even with the ascension of GM after 1930, however, and the rise of Japanese automakers in the 1970s, Henry's star continues to shine. Only Bill Gates made a serious run at Ford around 2000, but since then has been in decline as his interests have shifted from Microsoft to philanthropy. Were the graph to extend to 2013, I presume Steve Jobs might be in the top 5, but as you can see, Edison, Carnegie and Rockefeller are true, long-term heavyweights. Needless to say, America is very much a culture defined by the automobile and computer.
2. In 1900, Carnegie and Robert Fulton were the two most mentioned American innovators. Fulton peaked in 1910 (his steamboat perhaps overtaken by transportation innovations brought about by World War I) but Carnegie--famous for both his innovations in steel and in philanthropy--continued to resonate in literature right up to the present. At one point I tried both Cyrus McCormick and Samuel Colt who, along with Eli Whitney and Fulton, represented the four horseman of American innovation in the first half of the 19th century. Neither had a remarkable presence at any point in the graph.
3. From 1970 until about 2000, Thomas Edison (still #3) had a steady rise in mentions, possibly in tandem with American business's current infatuation with the culture of innovation.
4. Jack Welch, the former CEO of GE, continues to be a visible presence in literature. His recent decline, like Gates', is undoubtedly a function of his retirement. Note, however, that except for Jobs (and a tiny uptick for Zuckerberg), all of our innovators are in some decline. This general slump in long-term heavyweights could represent a fragmentation in business reporting, with America's press now focused on the up-and-coming technology innovators in Silicon Valley. It might also represent a more global reporting of business, with the old American innovators competing for press-time with an entire world of new talent. The result is that no single young innovator can command much press on his or her own, but in aggregate they might be stealing significant "print share" (and, therefore, business "mindshare") from the Old Guard.
Just for fun, I ran the Ngram again to compare Henry Ford with other American luminaries.
I then pulled the lens back one more time, just to get a true sense of proportion. This chart is considerably beyond my pay-grade, so I leave the interpretation entirely up to you!