A que te refieres con enviar y que no salga el error? Es bastante sencillo:
Formulario:
Código HTML:
<form action="tupagina.php" method="post">
<input type="text" name="valor1" />
<input type="text" name="valor2" />
<input type="submit" name="enviar" value="Generar" />
</form>
tupagina.php:
Código PHP:
<?php
include ("jpgraph.php");
include ("jpgraph_line.php");
$ydata = array($_POST['valor1'], $_POST['valor2'] );
$graph = new Graph(350, 250,"auto");
$graph->SetScale( "textlin");
$lineplot =new LinePlot($ydata);
$lineplot ->SetColor("blue");
$graph->Add( $lineplot);
$graph->Stroke();
?>