Show me a circuit where a transistor is used as a variable resistor, and we will help you understand it. I have seen FETs used as variable resistors, but not transistors.
FET = Field Effect
transistor LOL!
Anyways, I think all transistors can be used as variable resistors. Their resistance is dependant on temperature. Just make a multistage transistor amplifer and use an LED as the output. The brightness depends on the pressure applied to the "BASE" pin. The more powerful the amplifier, the less pressure you need to light up the LED completely.