Data approximation on graph mostly controlled by 2 constants:
ZBX_GRAPH_MAX_SKIP_CELL and ZBX_GRAPH_MAX_SKIP_DELAY.
So, if data absent period greather tahn this settings igraph just will be cuted to parts.
Otherwise line approximated.
So, my suggestion draw approximation which is not real monitoring data for distinguish. For example it may be dashed line of same color. Or line with lighter color.