QTreeView Checkable Item

[fruzzo]

Another question about qtreeview...
I created a qtreeview and inserted a checkable item as following:

MyPlot * plot = new MyPlot;
QStandardItemModel * model = new QStandardItemModel;
QStandardItem * item = new QStandardItem(plot->title().text());
item->setData( QVariant::fromValue(plot), PlotRole);
item->setSelectable(false);
item->setEditable(false);
item->setCheckable(true);
item->setCheckState(Qt::Checked);
model->appendRow(item);
myTreeView->setModel(model);

MyPlot * plot = new MyPlot;
QStandardItemModel * model = new QStandardItemModel;
QStandardItem * item = new QStandardItem(plot->title().text());
item->setData( QVariant::fromValue(plot), PlotRole);
item->setSelectable(false);
item->setEditable(false);
item->setCheckable(true);
item->setCheckState(Qt::Checked);
model->appendRow(item);
myTreeView->setModel(model);

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...and I've connected the model dataChanged signal to the following slot:

void MyWidget::onDataChanged(const QModelIndex & topLeft, const QModelIndex & bottomRight )
{
	QVariant variant = topLeft.data(Spada::PlotRole);
	if (!variant.canConvert<MyPlot *>())
		return;
 
	MyPlot * plot = variant.value<MyPlot *>();
 
	variant = topLeft.data(Qt::ChechStateRole);
	if (variant.canConvert(QVariant::Int))
	{
		Qt::CheckState checked = (Qt::CheckState) variant.convert(QVariant::Int);
		if (checked == Qt::Unchecked )
			plot->hide();
		else
			plot->show();
	}
}

void MyWidget::onDataChanged(const QModelIndex & topLeft, const QModelIndex & bottomRight )
{
	QVariant variant = topLeft.data(Spada::PlotRole);
	if (!variant.canConvert<MyPlot *>())
		return;

	MyPlot * plot = variant.value<MyPlot *>();

	variant = topLeft.data(Qt::ChechStateRole);
	if (variant.canConvert(QVariant::Int))
	{
		Qt::CheckState checked = (Qt::CheckState) variant.convert(QVariant::Int);
		if (checked == Qt::Unchecked )
			plot->hide();
		else
			plot->show();
	}
}

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Now, when I check/uncheck the item checkbox the slot is called but the checked variable is always Qt::PartiallyChecked...why?
Is there an error in the posted code?