There are 128.45 million households in the US. This graph explicitly round to the nearest whole percent. To have a percentage of 0, there could still be 642,141 households.
Edit: Actually I misread the stat. Of our 341.9 million people, 1.7 million might belong to a household making above a million dollars to still be approx 0%.
Yeah. I mean there is no figure like it on the graph but if something was like 99.6 it shoudl do >99 and not 100. 0 and 100 should be exceptions to the rounding rule.