In last sql query we use select for multiple itemids and then we calculate min clock for specified itemids.
We can simplify query - use MIN(clock) and select min clock at once.

For example:
Index: graphs.inc.php
===================================================================
— graphs.inc.php (revision 29944)
+++ graphs.inc.php (working copy)
@@ -266,10 +266,9 @@
}

$res = DBselect(

• 'SELECT ht.itemid,MIN(ht.clock) AS min_clock'.
  + 'SELECT MIN(ht.clock) AS min_clock'.
  ' FROM '.$sql_from.' ht'.
• ' WHERE '.DBcondition('ht.itemid', $itemids).
• ' GROUP BY ht.itemid'
  + ' WHERE '.DBcondition('ht.itemid', $itemids)
  );
  while ($min_tmp = DBfetch($res)) {
  $min = (is_null($min)) ? $min_tmp['min_clock'] : min($min, $min_tmp['min_clock']);