What's the significance of the location of the bubbles, how they're drawn? Are they separate datasets?

It says the x-axis is the 12-month volatility. As we move along the x-axis and have assets with increasing 12-month trailing volatility, we expect to see increasing returns on a 1-month time frame. This makes sense, higher volatility higher returns.

The fact that the bubbles (even silver) have show up in colored clusters gives us an idea of the how each assets is grouped in different volatility ranges? OP halp