You can define the x-axis values for your calculations in the Query administration form. (Click CRM> Setup> Sales management> Query administrationand then click the Allowed fieldsbutton.)

In the Allowed fields form, you can select the fields that you want to allow on the x-axis only, the y-axis only, or on both the x and y- axis.

Fields that will be plotted on the x-axis must be a form of data such as dates, customers and vendors, or employee names. Methods, such as revenue and quotations-won, can only be plotted on the y-axis.