This option is controlled through the Format (Caption) menu.
Captions are rows without data. Normally the value set label is used as a caption before the stub group and the caption text box is restricted to the same size as the other stub text. However the caption text box can be widened to the width of the table by using its Span fields option.
Select the "Span Field" option for the Format (Caption) menu.
Note: The horizontal alignment was also changed to "center" to demonstrate the size of the associated text box.