I did that one with weights and springs. It took a bit to shape the weights and get the weight right to do it. Lucking I had a local company make up a bunch of weights for that particular advance mechanism so I can play with a couple of spare weights, get what I want and then get that shape and size on the factory weights.
Then I keep them as pairs with numbers etched on them so I can log them in a notebook. Then when I need a similar curve I can pull them out and start with those.
If someone needs an ignition or is upgrading I tell them to buy a programmable box. It is much easier and quicker to ride the dots than it is to screw with the weights and springs.