Unfortunately automation is something known to be a little lacking in Renoise. See this thread for a basic enhancement and maybe some ideas into how we would like it improved to start.
But you do have three modes. Points, Linear and Cubic. Points will let you have instant changes when you want, Linear and Cubic are two different curves between the points (remember a straight line is also a type of curve.) Unfortunately you can not mix different types in one automation window and without the two values per point requested in the above thread you can’t have a ramp and an instant change in one automation window (have to make it different, quite possibly weird sized patterns or use LFO and Reset to get around this in a messy way.)
Personally I would love to see Bezier Curve functions in the automation!