fix Nullable.get_ is deprecated ? #65
Description
.dub/packages/ggplotd-1.2.1/ggplotd/source/ggplotd/guide.d(398,51): Deprecation: function std.typecons.Nullable!(RGB!("rgb", double, false, cast(RGBColorSpace)0)).Nullable.get_ is deprecated - Implicit conversion with alias Nullable.get this will be removed after 2.096. Please use .get explicitly.