Data approximation on graph mostly controlled by 2 constants:
ZBX_GRAPH_MAX_SKIP_CELL and ZBX_GRAPH_MAX_SKIP_DELAY.

ClineGraphDraw.php:

				if ($cell > $delay) {
					$draw = (boolean) ($diff < (ZBX_GRAPH_MAX_SKIP_CELL * $cell));
				}
				else {
					$draw = (boolean) ($diff < (ZBX_GRAPH_MAX_SKIP_DELAY * $delay));
				}

So, if data absent period greather tahn this settings igraph just will be cuted to parts.
Otherwise line approximated.

https://www.zabbix.com/forum/showthread.php?t=14360
https://www.zabbix.com/forum/showthread.php?t=43405

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.