What’s the problem? The electorate is probably a reasonable sample of the population at large. Granted it would have been better to count adults only, but the point is reasonable - that voting is mostly ethnic bloc voting like it is almost everywhere else on the planet.
You can point out ‘ethnic block voting’ without making bogus use of graphs.
Putting two measures on one graph line like that and then claiming that the GOP and the population are most similar—when you’re comparing graphs of two different things—is idiotic and/or deceitful.