Increasing inequality is a problem I consider very important for the progress of mankind, but I just want to point out that in this discussion on this specific topic, the share of GDP is not what is important. The absolute number (in PPP) is what we want to look at.
The Gini coefficient will give you a measure for the statistical spread of GDP per capita and is used to measure inequality. [1] Taking the time series of the Gini coefficient and the GDP will show the growth for a percentile of interest.
I guess I'm searching for a graph that has time on the x-axis and (real gdp * % of income that went to the bottom z%) on the y-axis for 5 ~ 10 different values of z. And hopefully it trends upwards for all values. I'm just surprised this isn't something that economists have already thought of as it feels like it would be the most important GDP-derived metric. Or at least "GDP per capita on median" if that makes sense.
