Why was Dawn's (the spacecraft) flight path circular? In comparison to the twin Voyagers, Dawn's flight path circled the sun in an anti-clockwise manner. The map of its path shows its targets, Ceres and Vesta were in a trajectory that was close to a straight path, if Dawn had taken a clockwise direction, with a close flyby of Mars. I can't find a reason to show it was studying the space around Lagrange point 3 (in relation to Earth), as according to this flight path map. So what was the reason for a longer direction?
Dawn was not exploring L3. But instead it explored both Vesta and Ceres. In order to do so in a cost-saving manner, Dawn was also a mission demonstrator for an interplanetary mission equipped with ion propulsion.

Because of the low specific impulse of ion propulsion, Dawn cannot fly in a straight line, as this would imply a high initial velocity and the spacecraft couldn't deaccelerate in time. Therefore, Dawn started with low $v_0$ at Earth, and raised its orbit slowly and continuously, creating the spiral you see.