Hi Jennifer,
You have to switch the lines in XY scatter mode (from the chart type tab select line and then from the subtype combo on the right select XY scatter). Then go to Data and double click on the values - this will show the “value groupings editor” - you'll notice that it now allows you to enter a X value - this is where you should assign the date time column.
The general idea is that value / date-time pairs represent scatter charts as both the x and y coordinates can be irregularly spaced (as opposed to ordinal charts, where only the y values vary and the x values are equally spaced - like the standard bar chart), therefore you have to additionally provide an X value in date time format before assigning a date time axis.
Hope this helps - questions or comments - please feel free...
Best Regards,
Nevron Support Team