You need to right-click the X-axis and choose Format Axis. There is a set of options on the Axis Options tab under "Vertical axis crosses:" There you can tell it to have the Y-axis cross "At category number" and specify a number that is half of the count of data points. You probably also want to position the axis so that it crosses on a tick mark.
That said, you will probably get better results with an XY Scatter chart, where it will cross at the intersection of the zero values by default.