lib/rrd.php question

[Int]

Could anybody explain?

lib/rrd.php
============================
/* override: graph end time */

if ((!isset($graph_data_array["graph_end"])) || ($graph_data_array["graph_end"] == "0")) {
$graph_end = -($seconds_between_graph_updates);
}else{
$graph_end = $graph_data_array["graph_end"];
}

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Why cacti use $seconds_between_graph_updates for "end" parameter of rrdtool graph?

i think it would be better to use "now"
instead of -($seconds_between_graph_updates)