Answer: The problem resides in the jpgraph.php file: Line 5791 and 5792.
If you take a look on that lines you’ll find this:
$this->use_anti_aliasing = $aFlg; imageantialias($this->img,$aFlg);
To solve the bug just change those lines to these:
$this->use_anti_aliasing = false; imageantialias($this->img,$aFlg);