When building a Capacitive touch slider like the picture shows, I know when finger tip touches the metal slider it would create a apparent sensor capacitance is increased by the introduction of a parallel path to earth through human body.
But how could the exact position of the finger tip on the plate be calculated?
The capacitive sliders are usually done using two capacitive plates, both referred to ground (not in mutual capacitance). The trick is drawing them sloped (triangular, not rectangular). When you touch the slider, in that position there will be usually one bigger and one smaller plate (except right on the middle), so you'll have two capacitance values. Some math will give you the estimated position.
