In the following animation you can see an example how selecting different measures and dimensions cause different stacking behaviour in the chart.
First we set the x channel to a dimension and an arbitrary constant value to illustrate stacking. (The dimension divides the bar to equal sections along the x-axis.) Then by changing the constant to a measure the markers’ width change according to the values. Finally when the dimension is removed the bars get ordered by the measure.
Note that when no dimension is present then the markers are measured from the origo.