Unfortunately only for line-charts there is the possibility to use xy-type. When
you want to create column-charts, whose categories are numbers, lets say years,
and the years don't have constant interval, you get a chart, which doesn't show
very well the facts.
Even if you have categories, that aren't numbers, you may want to get different
gaps between the bars. Therefore it is necessary to add another feature beside
data and categories to control the gaps between the categories. If categories
are years, this new "axis range" could contain exactly the same data. 
For a column of string-categories one could assign another column of numbers to
control the spacing. 
If user wants to get same spaces, one doesn't need this new feature. (Make
todays behaviour default.)

See bad and better example in attachements...