The acceleration $a$ is the derivative of velocity $v$ with respect to time $t$: $a=\frac{dv}{dt}$. On a $v-t$ graph, this derivative corresponds to the slope of the line. If the graph is a straight line, its slope is constant. If the slope is positive, the derivative is positive. Therefore, a straight line with a positive slope on a $v-t$ graph indicates a constant positive acceleration.