How to control the speed of a rotation

Hi !

I'm currently trying to control the rotation speed of an svg image. For the moment I wrote this :

rotation = ofGetElapsedTimef()*100*rotationSpeed;

Where rotation is my angle and rotationSpeed is a midi knob which return a value between 0 and 1.
So basically, it should not rotate the svg when set to 0 and make it rotates faster and faster when I turn it.

My svg is well rotated, but when I turn the knob it becomes Glitchy, values are skipping.

I guess I need to introduce a vector like

rotation = oldRotation + delta*(newRotation-oldRotation);

or something like this, but I can't figure it out.

Thank in advance for the help

Cyril