Window and step parameters for a graph

There is a condition that "Steps per window" in a graph parameters should fit to "Window" size. But when a sequence length is too short, than this condition is not hold, e.g.
1. Let "window = 30, step = 10" by default
2. Open sequence with length=12
Result: "window=12, step=10"

This behaviour should be fixed. Maybe the parameters should be selected depending on the size of the sequence.
And also it would be nice to introduce "Step" instead of "Steps per window".