With a first look at that chart, what it seems to truly reflect is the size of the middle class in each nation.

Wouldn't a better solution be to increase the size of the middle class rather than decrease the size of the upper class?

Also, among the western european nations and the US, I suspect a strong correlation between immigration rates and the data on the chart.

Problem is, a strong middle class usually rests on the strong provision and spread of certain goods. Some of these are private goods, like owning one's own house, but many of them are public goods: liveable environment, public transit, public schools, public universities, public health and health-care, etc.