By default columns within Data viewer will represent the date and periods you have selected - typically days, weeks or months. However, there may be times when you're rather have the columns represent a data type such as teams or job types. This is where setting a cross tab comes in to use.
Rather than selecting to 'add' a data you can select 'Set cross-tab':
This will then display that selected data as the headings, not the period of time selected: