diff --git a/htdocs/core/class/dolgraph.class.php b/htdocs/core/class/dolgraph.class.php
index 57f0bca4e36..6ef5b69ba3e 100644
--- a/htdocs/core/class/dolgraph.class.php
+++ b/htdocs/core/class/dolgraph.class.php
@@ -1211,7 +1211,7 @@ class DolGraph
 					$tmp = str_replace('#', '', $this->datacolor[$i]);
 					if (strpos($tmp, '-') !== false) {
 						$foundnegativecolor++;
-						$color = '#FFFFFF'; // If $val is '-123'
+						$color = 'rgba(0,0,0,.0)'; // If $val is '-123'
 					} else {
 						$color = "#" . $tmp; // If $val is '123' or '#123'
 					}