Width

This scrollbar sets the width of the smoothing window. This can be in
either fixed distance or a fixed number of points. 

Smoothing is done on the world data scale of -32768 to 32768.

In terms of distance the smallest width provided by the scrollbar is
max(1000,step*10), and the maximum is 32768/3~10923.

In terms of number of points the smallest is 2 and the largest is
nrows_in_plot/6.

The step size does depend on the actual window size so the two
scrollbars don't always work independently.