Rather similar to Iresine, this plant comes from Central and South America and belongs to the same broad plant family (Amaranthaceae). It grows to around 50-100 cm tall and quite wide (to 1 m) with stunning aubergine-coloured pointed leaves. It grows best in a sunny spot with adequate moisture. Its flowers are insignificant. It gets shabby by the end of winter in Sydney, but it will survive as long as it is not cut back until around the beginning of September. It tends to send out long angular stems, so it can be trimmed back periodically through summer to keep it neat. A more compact version with smaller leaves and a denser habit is marketed as 'Little Ruby' (ht 30 cm, width to 90 cm). I really like this smaller version though I find it looks terribly ratty by the end of winter in my garden. It may do better in warmer areas. I tend to pull them out at the end of autumn and start again with a fresh plant in spring.
Like all dark-leafed plants it makes an effective contrast for most other colours, whether they are hot reds, oranges and yellows or cooler pinks, blues, purples, white or greens. I also have it growing nearby a burgundy-coloured Pennisetum and silvery Plectranthus argentatus, a combination which looks good all summer long. There are other lovely cultivars of this Alternanthera, some with pretty pinkish markings, but I have found they die in winter in my garden so I have not persisted with them. This plant is easily propagated by cuttings or the layers which it often produces during the growing season.