From your screenshot, it looks like you don't actually have an x-axis. You have a y-axis for sales and column headers for each date.
Here's the difference:
The date field on Columns is blue which indicates that it is discrete and thus defines headers (individual values like "April", "May", "June" -- which you can even reorder if you want). A green field is continuous and defines an axis (either numeric such as 1 to 100 or dates such as April through June). Unlike headers, you cannot reorder the values (e.g. 98 cannot come before 2), but you can edit the axis to change things like the range.
In Tableau, dates can be either discrete or continuous and you can decide which works best based on your case. To change the field, simply right click it (or use the little drop down arrow) and change it. The top date parts are discrete by default and the bottom date parts are continuous by default (but you can even change that if needed).
Try the highlighted one below: