Trend Value with Intentional N/A or Null Data

Hello All,

I am interested in creating a pseudo value that would only have a numerical value in some instances and be nothing otherwise. I want to be able to trend this value, so how do I create this variable's value during this null/nothing period so the trend system shows it as no data instead of 0 or some exstapalation from previous 'good' value?

Thanks

Andy