large values can break graphs and cause division by zero.
for example, item history displays the value as :
1.1111111111111E+16

graph on it fails and php log contains :

PHP Warning: Division by zero in /usr/local/apache2/htdocs/trunk/include/classes/class.cchart.php on line 749, referer: http://localhost/trunk/history.php?action=showgraph&itemid=22520