If the lines on the plot are not parallel, an interaction may exist. An interaction indicates that the means of the Y data at the levels of the first X variable are different at each level of the second X variable. The more nonparallel the lines are, the greater the strength of the interaction.
The following line plot shows the mean strength of fibers produced by 4 machines and 3 operators. The nonparallel line associated with operator 1 on machine 2 suggests a possible interaction. This combination produced an unexpectedly high mean strength that may warrant a follow-up investigation.
Although you can use the line plot to display effects and interactions, you should perform the appropriate statistical test to evaluate the statistical significance of any effects or interactions that you see. If the interaction effects are significant, you cannot interpret the main effects without considering the interaction effects.