I'm trying to achieve a line graph with transparent background, but so far, I can only set the background to a specific color. Since it's PNG, it should be possible though.
$linePlot = new LinePlot($plotData);
$linePlot->SetFillColor('#61a9f3@.6');
$graph = new Graph(300, 100);
$graph->SetScale('intint');
$graph->SetAlphaBlending();
$graph->SetMarginColor('black:1');
$graph->SetFrame(true,'black', 0);
$graph->Add($linePlot);
$graph->Stroke();
Result
Not transparent, but black
$graph->SetTheme($theme_class);
$graph->img->SetAntiAliasing(false);
$graph->title->Set('Filled Y-grid');
$graph->SetMarginColor('White:0.6');
$graph->SetFrame(true,'White:0.6',1); // The '1' here seems to be irrelevant
// I've tried much larger numbers with no
// change. This is supposed to be frame
// width (in pixels).
$graph->SetBox(false);
Reference: change margin color for a jpgraph