A: The histogram chart uses the current colour attributes to subdivide the data.  This allows quick visualisation of the relative contributions by different subgroups to the overall sample count.  The stacked mode merely displays the coloured bars on top of each other, and thus the totals for each bin would be the same as for a histogram without colour groups (ie only ‘default’ set in the colour attribute dialog).  The layered view plots each group 1 behind the other, with the colours made semi-transparent, and is generally only useful for 2 or 3 groups.  The number of bins can be modified using the up and down arrow icons, while individual colour bins or the entire bar can be selected for attributing using the top two icons.