Here's the explanation from Data Pointed:
That's a line for every year from 1913 onward, sized and colored by the tax burden: the amount of tax due relative to the long-term average at each income level. Above-average burdens appear thick and red and below-average thin and blue.
So, for example, if you want to look at the present day, look at the line along the very bottom of the graphic.
You see that the tax burden for people with very low incomes is lower than it's been over the past century. The burden for people with middle incomes is slightly higher. And the burden for people with high incomes is significantly lower.
As you move up the graph, you go back in time. You see that taxes for everyone were significantly higher than average in the '40s, '50s and '60s. Go back further in time, and you see that taxes were lower for almost everyone before World War II.
Hat Tip: Felix Salmon