After 3 years of silence I'm not sure if you're still interested in having feedback on your request, but anyway here it goes...

I personally think that a "clean" fallback to FS_NORMAL should ideally be handled by JPGraph, rather than them just throwing an exception and aborting the graph's generation. That said, JPGraph development seems dead for over 2 years, so I see little chance of anything happening in this area.

Of course we can also work around this in MantisBT either by sticking to FS_NORMAL, or by defining a function to apply a font style in graph_api.php, like this (tested with JPGraph 3.5):

/**


Allows a 'safe' use of the specified font style, i.e. reduces the risk of an
exception being thrown when the graph is drawn in case the style is not
defined for the given font. This is done by falling back to FS_NORMAL if the
style does not exist.
example: $graph->title->SetFont($font, graph_font_style( $font, FS_BOLD ));
@param int $pfont JPGraph font family (FF* constants)
@param int $pstyle Font style to use (FS* constants), defaults to FS_BOLD

@return int Font style to use in SetFont() calls

*/

function graph_font_style( $p_font, $p_style = FS_BOLD ) {

$t_ttf = new TTF;
try {

$t_ttf->File( $p_font, $p_style );

}

catch( exception $e ) {
Assume failure due to non-existent style, fall back to FS_NORMAL
# other exceptions will be caught and displayed when the graph is drawn
return FS_NORMAL;
}
return $p_style;

}

Then, various calls to SetFont() can be replaced as follows:

$graph->title->SetFont( $t_graph_font, FS_BOLD );
$graph->title->SetFont( $t_graph_font, graph_font_style( $t_graph_font, FS_BOLD ) );

Let me know if that works for you, and if you complete a revision of the MantisGraph plugin with Asian fonts compatibility, upload it here and I'll see if I can add it to core.