I have a stacked bar graph that is for a historian, with centuries as the categories. The prof. would like the labels (400 BC, 300 BC, etc.) to be diagonal, centered on a tick mark between categories, since the "stack" refers to the time during a given century (i.e. 400-300 BC), not a single year (i.e. 400 BC or 300 BC). Is it possible to move the tick-mark labels on the X-axis?