The deep-purple forms of this plant are often grown in gardens, but they do not confine themselves to where we plant them. This one was growing out of a sidewalk in Beechview.
If we have misidentified this species of Amaranthus, we may plead in extenuation that the taxonomy of the genus is confused, and in the USDA PLANTS database most of the other possibilities are reduced to forms of Amaranthus cruentus.