A small cart is released from rest at time \( t = 0 \) and moves down a straight track with constant acceleration \( a \). A motion sensor measures the speed \( v \) of the cart at different positions \( x \) along the track, where \( x = 0 \) is the starting position. The data collected is used to create the graph shown of the square of the speed \( v^2 \) as a function of position \( x \). Based on the graph, what is the acceleration of the cart?