Seems like it makes the point perfectly well. You are implying that smaller countries have fewer nukes because of US sanctions, but it could easily also be that those countries are simply smaller. Where it mattered, the US's main enemy, the US regulation did nothing to stop Russia from building as many nukes as they wanted to.
Also, the US has significantly less power worldwide than it did for most of that chart. Today, arguably, China exerts as much power as the US. American's always love to brag about how exceptional the US is, but often that isn't as true as they think and certainly won't be true for the long run.
Smaller countries like China and India? Population-wise they're larger, and area-wise they're not two orders of magnitude smaller. My point is that the chart doesn't really show nukes "spreading around the world" but concentrated almost entirely in two countries. Maybe the US policy did nothing to help it, but for all we know there would have been plenty of other countries with thousands of nukes as well without it. I'm not arguing that the policy was effective or not, just that I don't see how that chart is enough evidence alone to conclude one way or another.
Minor comment, but I don't get where some people take that China is smaller than the USA. China is the 2nd largest country in the world and its landmass is ˜2% larger than the US (including Alaska).
India though, even as 7th in the world, is smaller than the US, with about 32% of its area.
Population-wise India has now surpassed China, and both beat the US (3rd in the world) by over 1B people each.
Also, the US has significantly less power worldwide than it did for most of that chart. Today, arguably, China exerts as much power as the US. American's always love to brag about how exceptional the US is, but often that isn't as true as they think and certainly won't be true for the long run.
Long term planning needs to avoid such arrogance.