To position the X axis at the bottom of the chart with negative values in Microsoft Excel 2007, follow these steps:

1. Select the chart by clicking on it.
2. Right-click on the chart and select "Format Chart Area" from the drop-down menu.
3. In the "Format Chart Area" dialog box, click on the "Axes" tab.
4. Under the "Axes" tab, select the X axis and click on the "Horizontal axis crosses" drop-down menu.
5. Select "Axis value" and enter
   Formula:

   "0" 

   in the input box.
6. Click "Close" to close the dialog box.

This should position the X axis at the bottom of the chart with negative values. If you have any further questions, feel free to ask.