Attack and decay on StepSlider (lerp?)

Hi guys.

I'm trying to build a StepSlider, that would allow me to set different ramps on the value change, depending on if the new value is going to be lower or higher than the previous one. I would also like the ramp to be able to be longer than one step of the StepSlider. Any thoughts?

I would normally approach this with some kind of linear interpolation (lerp), but the Lemur scripting language doesn't seem to support it out of the box.

Thanks for your help!