Mosaic plot is a special case of stacked bar chart where you have the proportionality (Mosaic Plot). If you don't want that you can always use stacked bar chart. I'm not really fan of having proportionality enabled by default in Oneway but that is what we currently have and we can change that in preferences if we want to
Also I think contingency is supposed to create a mosaic plot -> no need for option to remove proportionality.
-Jarmo