From what I've read about stacked bars charts, if you have two values, such as 50 and 100, that are going to stack, then on the y axis, one of the values should go from 0-50 on the y axis and the other should go from 50-150, or from 0-100 and 100-150, but the example in the tutorial puts both bars at 0.

Another way to put this: you should be able to see both bars even if you change the order in which they are drawn, but if you change the order in the example, then you only see the taller bar, as the following screenshot shows: