The wonderful thing about GeoGebra is that you can whip up an applet from scratch within an hour just before your lesson and use it immediately to demonstrate a concept involving interdependent variables. I was motivated to do this after trying to explain a question to my IP4 students.
The RGB colours of the thermistor reflects the temperature (red being hot, bluish-purple being cold)
This was done to demonstrate the application of potential dividers involving a thermistor and a variable resistor. It can, of course, be modified very quickly to introduce other circuit components.