JpGraph offers two ways of handling null values (or discontinuities) in the data. You can either have a "whole" in the data or the line may be extended between the previous and next data point in the graph.
If the data value is null ("") or the special value "x" then the data point will not be plotted and will leave a gap in the line.
If the data value is "-" then the line will be drawn between the previous and next point in the data ignoring the "-" point.
The following example shows both these possibilities.