Identify a plant

A few years back at my old house I planted a plant in my rockery, so it must have been in the Alpine plant section, I was amazed at how tall it grew, maybe a Meter high, with long strong stems, the flowers were dark purple, the shape was not unlike the growth of a tree, it had straight up stems then divided up into 4 or more, it was strong, but it did require a cane as it grew, do any of you know the name or any suggestions, as I would love to get one for my new home.