To define range B3:E3 as the horizontal axis label values, enter the range B3:E3 as the axis labels in the 'Custom' text box and then select apply to effect the changes.
How to define a range in a sheet
Dragging and dropping is the most basic method of defining a range in a sheet. Click and hold the top left cell of the desired range, drag your mouse to the bottom right cell, and release. The cells you have selected will be highlighted, indicating the range.
You can also directly type the cell references of the first and last cells in the range, separated by a colon or use formulas to dynamically define a range based on certain criteria.