What is this please?

And I know it's a bit lacking in foliage, sorry!

I pinched a few of these from a friend's garden last year, and this one has survived. When I pinched it, it had whorled leaves, arranged in sets of four in an X shape, one set directly above the other. Very striking plant, almost looks like someone's made it.

The leaves (when they are there :P) are quite grass-like, long and narrow, fairly rigid, darkish green with white centres. Single stemmed and upright plants. They were self-seeded in my friend's garden so she has no idea what they are.

This one has suffered a bit from lack of water, I got a bit lax with my smaller pots but back on track now and these flowers have appeared

So, anyone got any idea? I've been trying to find it through plant ID keys but I'm not getting anywhere.

 

I think it's some kind of euphorbia, or spurge nikirushka. Not sure which one, possibly common spurge (euphorbia helioscopia).

Just a note of caution; wear gloves when you handle it, as the milky sap inside the plant can cause skin irritation for some people.

 

That's it! Thank you very much, both! I will be careful with it, I'll have to plant it properly this year I think so I'll bear the sap in mind.

Thanks again.