The number of guns could be owned by a smaller and smaller proportion of the population, which does actually seem to be the case. After a quick google it seems that in 1970 or so, over half of the United States housholds had a firearm, whereas that's reduced to about a third nowadays. If you had plotted this line on the graph, well, the same point wouldn't be made. I don't really care how many guns collectors have, murders tend to be done with one gun. The relevant figure is therefore how many people or households have guns, not really how many privately held guns there are.
In general, though, it's just bad form to plot one figure that's normalized to population size, with another that isn't.