Generating graphs in an external program (in this case, gnuplot) may cause problems. This is a solution hacked together in the late hours of night.
<?php
// get the values from, say, a database
$vals = "1 2\\\\n4 8\\\\n"; # etc
// construct the gnuplot_call variable, something like:
$gnuplot_call = "echo \"set term png; plot '<echo -e \\\"$vals\\\" ' ti 'Updated with' with line ; \" | gnuplot";
ob_start();
passthru($gnuplot_call, $gnuplot_call_output);
$image = imagecreatefromstring(ob_get_contents());
ob_end_clean();
imagepng($image); # display
?>