When a linear equation is in the form y = mx + c, the c, or constant, is the intercept on the y axis, meaning it crosses the y axis at (0, 1).
The gradient (1/3 in this case) is how much the y increments (or decrements) per increase of 1 of the value of x.
This would mean that there would be one point at (0, 1), and another at (3, 2). Draw a line from these two points and beyond, and that is the graph sketched.