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I think I finally understand the problem chart:axis-type="date" is suppsed to address:
Some charts, for example the chart:bar class, have an axis that is predefined to be either a categories axis (i.e. category labels are evenly spaced and the corresponding values are shown at these evenly spaced locations) or a value axis (i.e. values are shown at their relative locations).
For example for a barchart with x-values that are dates (years ), e.g. 1950,1980, 2000, 2010, with a category type axis the 4 bars would be evenly spaced as if at values 1,2,3,4 with these points on the x axis labelled 1950,1980, 2000, 2010. If we had the same chart type but with a value type x-axis the bars would not be evenly spaced since the difference between 1950 and 1980 will be three times as large as the difference between 2000 and 2010.
I believe Regina's suggestion is to obtain the second chart by labelling flagging the second axis as chart:axis-type="date".
To me all of this has nothing to do with dates. These values 1950,1980, 2000, 2010 simply differ from other categories like a,b,3,d that they are all numbers and so can be interpreted as relative locations on the x-axis.
So since there is nothing special about dates being involved (except that apparently Excel allows the second type of chart only for date values) I believe we should not make this date specific, but simply flag axis with their type (if it differs from their default type), e.g. chart:axis-type="values" or chart:axis-type="categories" or chart:axis-type="value-preference" where the last type means that if all values/categories given are numbers then the axis behaves like a value axes (placing items in their relative position) and like a categories axis otherwise.