Setting chart value to dates

I'm looking at a chart of monthly sales and trying to find a time trend. I've got the first column set to mm-yyyy and the second set to number



Jun-97 36
... ...
Jul-08 -19

Now the problem is that when I chart this data the chart displays the dates on the x axis and the values on the y axis. However, when I try to chanfge the select the scale for the x axis I can only select numbers rather than dates and when I do a regression line it comes up with the correct slope but a really wierd number (as if the x axis started at 3960 rather than jul-97

What should I do so I can set the 'x axis' to dates?