Try to put your graph script into one file (graph 300x200px):
...
$graph->Stroke();
And into another file:
$pdf=new FPDF('P','mm','A4');
$pdf->SetTextColor(44,44,44);
$pdf->AliasNbPages();
$trigger = $pdf->PageBreakTrigger;
$pdf->AddPage();
$pdf->Image('http://localhost/graph_script.php*', 10, 10, 25.4/150* 300, 25.4/150*200);
$pdf->Output('oferta_numer_.pdf', 'D');
*Absolute path to script (I`m not sure relative path works)
Go to http://localhost/another_file.php