Identification

This appears in a small corner of a border every year. Self seeds.

It's not a problem plant and it's actually quite nice. It soon gets swamped by Rudbeckias and Dahlias though. I've never seen it get over 8 inches high.
I potted some up in the spring and snipped the tips out. (Milky sap)

They have bushed nicely and are 7" high and look like thay could go more.

I always assumed they are a type of Euphorbia but i can't find the variety name on search.
Any ideas?

Thanks.