Define two lines for our linegraph, A and B. A has two points (0,5)(10,6) B has two points (1,3)(3,4)
Setting line B to fill will fill the space under and between (1,3) and (3,4). However, the space from 0 to 1 and 3 to 10 on the x axis will be completley filled in up to the max y range. The space before and after the line are filled in completely.