Code

void MergeCells(TableNode table, int column)
{
	int startIndex = 0;
	while (startIndex < table.Rows.Count)
	{
		int span = 1;
		while (startIndex + span  < table.Rows.Count &&
			table[column, startIndex].Text == table[column, startIndex + span].Text)
		{
			span++;
		}
		if (span > 1)
		{
			table[column, startIndex].RowSpan = span;
			table[column, startIndex].TextFormat.FormatFlags =
				StringFormatFlags.DirectionVertical;
		}
		startIndex += span;
	}
}

var t = diagram.Factory.CreateTableNode(10, 10, 30, 100, 3, 20);
t[0, 0].Text = "test 1";
t[0, 1].Text = "test 2";
t[0, 2].Text = "test 2";
t[0, 3].Text = "test 2";
t[0, 4].Text = "test 3";
t[0, 5].Text = "test 4";
t[0, 6].Text = "test 4";
t[0, 7].Text = "test 5";
MergeCells(t, 0);