Michael, that graph means very different things depending on what the unlabeled Y axis represents. Is that per capita or total number homocides? I also don't know what was and what was not considered a 'homocide' at different times....certainlty 1900 forensics wasn't able to detect poisons, fingerprints were maybe just starting to be used, and staging a suicide or accident was much much easier than it is today.
But the main point is that in 1900 there were about 76 million people in the us...and today there is over 300 million...looking at the graph and comparing the pre-1900 numbers with todays numbers, it looks aproximately like the same 4x increase in population.