From the description & pic check Indigofera miniata.

Can you show us the leaves?

Meanwhile look at Lathyrus annuus 'Hotham Red' and see what you think.

I am sure its Lathyrus annuus Hotham Red or at least Lathyrus annuus. I don't have a photos of leaves because back in 2001, I did not realize the importance of saving a photo of the entire plant. The vine is stringly, and the flower is small, but I had a new macro camera so I took a macro of the flower. Thank you for solving this. I would like to try it again, beause they save water compared to sweet peas and they also can take more heat. I am guessing the less fleshly vine will have water and take more heat. It is over 100 each day in the summer here for three months, or so it seems to feel like that everyday.

I found a great site with photos of these types of flowers. The big difference between them and Lathyrus odoratus is the vines are very thin, so they don't shade out other plants planted underneath them. I grow my sweet peas up against the fence, but they the bushes got so big they took over the entire area, and got powderly mildrew. I was growing them in San Francisco in the bare ground. But, I knew a lady who was a better growing, she lived near me, and she grew hers in containers that were like long wooden boxes with drainage holes. She created a network of strings that were attacked to stakes. I don't know if she was thining out the excessive foliage on the sweet peas, but hers always looks fantastic. Her bushes were tamed and her flowers in the hundreds everyday.

German webpage but lots of photos are here

Roger Parsons Sweet Peas thinks my sweet pea is Lathyrus gorgonei instead of Hotham Red, I have ordered some Lathyrus gorgonei seeds.