Instead of single column, have 2 columns in source table. In one of them, use NA() to return an error code, when there must be gap in graph, and values otherwise. In another column, use NA() for those rows where you had values in 1st one, and some appropriate value otherwise. Have both columns display a line (coloured differently) in your graph. I don't remember exactly how, but you can have a gap in line graph for N/A's in table.