Defines when to display a gap in the graph, together with the gapUnit option.
In case when dataGrouping is enabled, points can be grouped
into a larger time span. This can make the grouped points to
have a greater distance than the absolute value of gapSize
property, which will result in disappearing graph completely.
To prevent this situation the mentioned distance between
grouped points is used instead of previously defined
gapSize.
In practice, this option is most often used to visualize gaps in time series. In a stock chart, intraday data is available for daytime hours, while gaps will appear in nights and weekends.