I remember my dad telling me years and years ago about an arbitrary function generator that they used at university, where you had an oscilloscope with a cardbo

I remember my dad telling me years and years ago about an arbitrary function generator that they used at university, where you had an oscilloscope with a cardboard mask representing the function's graph in front of the screen, and a light sensor in a light-tight funnel that controlled the spot height. When light shone on the sensor it would reduce the input voltage, and so the spot would trace the top of the cardboard mask. This actually works quite well!