Both the locations where we link to this call plugin_page() to do a relative link ( the file was moved). As per below it works for me:

Do you have any custom plugins loaded?

http://127.0.0.1:81/plugin.php?page=MantisGraph/bug_graph_page.php

C:\mantis\git\mantisbt\plugins\MantisGraph\MantisGraph.php (1 hits)
Line 82: return array( '<a href="' . plugin_page( 'bug_graph_page.php' ) . '">' . plugin_lang_get( 'graph_bug_page_link' ) . '</a>', );
C:\mantis\git\mantisbt\plugins\MantisGraph\pages\bug_graph_page.php (1 hits)
Line 61: <form name="graph_form" method="post" action="<?php echo plugin_page( 'bug_graph_page.php' ); ?>">

I have no custom plugins loaded.

In the 1.2rc1 release, there is no bug_graph_page.php in plugins\MantisGraph\pages

Neither does the page appear on mantis's git repository:

http://git.mantisforge.org/w/mantisbt.git?a=tree;f=plugins/MantisGraph/pages;h=9a09ab077c74fe8ea6ec8e11bbb2d25e6e72d1e3;hb=HEAD

sigh, we git ignore the plugin dir