The answer to your question depends on what you mean by "exact".
Since no geometry drawn on paper or on a screen is truly exact, the answer is that any graph you make is not exact.
However, there are some problems for which the answer is difficult to discern, e.g when you want to get the point of tangency of a steep parabola or the point of inflection of a step curve.
In terms of systems linear equations it would be my opinion that all systems can be solved "exactly" by graphing provided you know how to interpret systems when the lines are parallel or superimposed.
