One thing you might want to check is to make sure you don't over expand a variable.
For example, if D is the value of variable vVar and in the expression you put $(vVar), you will get a null value while if you put just vVar you will get D.
If your variable is =sum(Sales) you call the variable without dollar-sign expansion to return one value, but if your variable is sum(Sales) then call the variable with dollar-sign expansion so that performs that expression in the chart.
In summary, call the variable without dollar-sign expansion when you want to use the direct value of the variable. Use dollar-sign expansion when you want to use the value of the variable to calculate something else.
