Oh well, this works:
foreach ($price as $k => $v)
{
$DataSet->AddPoint($v,"Serie1");
}
foreach ($date as $k => $v)
{
$v = mktime(0,0,0,$days[1],($days[2]),$days[0]);
$DataSet->AddPoint($v,"Serie2");
}
Don't know why the other didn't though, whatever. :p