That depends on what kind of visualization you want to make. YoY is a "loose" term that doesn't prescribe whether you stack or sequence the years. The way I suggested by stacking them I have used a set expression to single out the values for a particular year and making each month more easily comparable to corresponding months from other years.
I have only chosen Month as a dimension and then I had to single out each year by a separate Measure and the way to single each year out was by using a set expression:
Sum( { <Year={2014}> } Inv )
The outer { and } tells Qlik Sense that we want to do further specification on which values that will be considered for the Sum() in the field of Inv. The base selections already implied are determined by what is selected in the user interface and also by the dimensions that have be selected in the chart that the expression belongs to.
The < and > telles Qlik Sense that we will modify the set of values by specifying some field selections. The Year is the name of the field that will be used for further selection and the set of values denoted within the inner { and } is that values that should be included in the selection.
So in this way we have simply selected all values of the Inv field that is associated with year 2014 and they are summed up to create a resulting value which will be calculated for each combination of dimensions. In this case the dimension is Month. Thus we have created a set of calculated values for the year 2014 and every month that has has a corresponding sum of values.
