See GD::Graph.

transparent
If set to a true value, the produced image will have the background colour marked as transparent (see also option bgclr). Default: 1.

$my_graph->set(
  x_label        => 'X-Label',
  y_label        => 'Y-Label',
  title          => 'Title',
  do_stats       => 0,
  symbolc        => 'black',
  y_min_value    => -150,
  y_max_value    => 200,
  transparent    => 0,  # << add
);
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Also have a read of Use strict and warnings