If we commonly draw a distribution of data, we should be careful to chose appropriate classes, for instance,
1-2, 4
2-3, 6
3-4, 11
etc.
But, if we draw a cumulative distribution, classes are not necessary. For instance
1-2, 4
2-3, 10
3-4, 21
and still better:
1, 1
1.3, 2
1.4, 3
1.9, 4...