Major chart:class="chart:bar" is the type of charts that results in vertical column or horizontal bar charts depending on the value of the attribute chart:vertical. chart:vertical=true should give us horizontal bar charts and chart:vertical=false (the default) vertical column charts. So far that is well described.
The problem is that we only know that chart:vertical is an attribute to chart-properties, so it is part of some style of the chart family. It is not said to which element this style should be applied.
Off hand I would guess that the style should apply to the chart:chart element since that element carries the class information that we are considering a chart:bar. (Since the class can be overwritten by the series element for specific series, it also makes sense that the style attached to a series element could set chart:vertical, if it is understood that that only modifies that series. (MSOffice and Gnumeric are using this interpretation.)
SInce chart:vertical in its general description claims to specify whether the x-axis is vertical it would make some sense to apply that style to the chart:plot-area since that element contains all axes. (OOo seems to be using this interpretation.)
Moreover, if styles with conflicting chart:vertical are applied to chart:chart and chart:plot-area (or sany of othe rpossible elements), it seems to be completely unclear which value applies.