Set the variable resistor to 50 percent. The 10 K Ohm resistor is at 5 K Ohm which. The voltage is 2.5 Volts on both sides of the meter so the difference is zero. The ratio of the two sets of series resistors are the same. 1k in series with 1k and 5 K in series with 5 K. Move the resistance alittle and the voltage across one side of the meter is different so you see a voltage reading on the graph.
