Overlapping column charts are multi-series charts, with all single series vertical columns displayed clustered (instead on side-by-side), on a different row, on the 3D perspective. It's like each series' points are one behind the other. First series has all columns visible, in the front row. The other series have their columns partially covered by the series in front.
Our Global Sales demo charts show groups of product quantity totals sold in US, France and UK, in three different years. The multiple series are the US, France and UK countries. The values for each year are 2007, 2008 and 2009. Remark the horizontal X axis is a category axis: year values are distributed at equal intervals on the axis. Quantity value columns for each year are displayed one behind the other for each country, with a 3D perspective. All columns for a specific country have a different distinct color. First country uses the first palette color, second country the second palette color and so on.