Hi there,
Line breaks are entirely allowed and using one doesn't mean you are manipulating the axis.
If you however, for example, have a scale from 0 to 300 on the y-axis and 0 to 30 on the x-axis to graph out something like "The number of students present in a day for 30 days" and you know for a fact that there are only 50 students in class. By doing so, you are confusing the person looking at the graph and are making him think that there are very little children actually present. Now this is a case of Manipulating the Y axis.
-Robin Justin