The graph is missing but I guess you refer to the "Force vs mass" graph. In this graph, the acceleration is the slope of the curve, because the relationship between force F, mass m and acceleration a is (Second Newton's law)
F= ma
And in the graph, m is the x-variable, F is the y-variable and a the slope.
We can notice that there is a direct proportionality between F and a. So, if m is constant, when we triple the force (F' = 3F), the acceleration will also triple (a'=3 a). But since the acceleration is the slope of the curve, this means that the new slope will be 3 times the slope of the original curve.