he simulation has added a new feature, test charge polarity to simulate -e


an electron test mass moving through the point of minimum potential between Q2 = +3C and Q1= +1C. the theoretical path is one that the test charge electron moves and oscillates in the curved path as shown. The slight loop shape in due to the unbalanced forces at a far distance that deviates the direction of the electron to go slightly towards to the Q1 in this case. 

solution to  2012 A level H2 P2 Q7d(ii) by jeffrey toh. and his team. thanks for sharing!