Earlier quoted context omitted.
Do you understand the difference between “income” and “income share”?
The graph defines what it charts very well at the bottom. You can read it, and then if I am somehow still very wrong about the fact that income in the bottom 50% has gone down, or hasn't kept up if you'd prefer, then you can really stick it to me on this public forum about how I am wrong, and you are right, the only thing that matters on the internet.
That graph makes no statement whatsoever about whether real income in the bottom 50% has gone down, stayed flat, or risen over the 30-year period under discussion. For that, you’d look at a real income chart over the 30 year period, which would show that real income has increased over that span.
If incomes at the bottom rise by 20% vs 10% inflation and incomes at the top rise by by 100%, the income at the bottom went up in real terms, though the share of income at the bottom will have gone down.