Together with gapSize, this option defines where to draw gaps in the graph.
When the gapUnit is "relative" (default), a gap size of 5
means that if the distance between two points is greater than
5 times that of the two closest points, the graph will be
broken.
When the gapUnit is "value", the gap is based on absolute
axis values, which on a datetime axis is milliseconds. This
also applies to the navigator series that inherits gap
options from the base series.