This is phryma leptostachya or lopseed I saw in the Koishikawa Botanical Garden, Tokyo. The stem is so thin, the flower so tiny, and the color contrast between the white and the purple is so pretty!
What surprised me was the plant's name in Japanese is "Haedoku so," which means "Fly poison plant," given after the fact that the whole plant is poisonous and the poison is used in flypaper. I get it, but it's so unfair, isn't it😩!