Placing X and Y axis lines at 0 for a scatter plot with negative values?

Hello all,

I have switched to Numbers this semester and am really enjoying using it for my classes that require lab data to be graphed. I hit my first snag with this issue, though. I've been browsing Google and the Apple Support Communities for almost an hour and a half now to no avail.

My question: how can I create a scatterplot with the axis in the middle of the numberline, rather than the default bottom left version Numbers spits out regardless of what I set the Min Y value on?

Basically, I just want Numbers to do what Excel is doing here:

http://img838.imageshack.us/img838/6606/screenshot20110925at005.png

I want Numbers to put the axis line at 0, in between the negative and positive values, rather than it's default setting.

In order to complete my homework, I need to extend a trendline (I know how to do this by adding points) so that it intersects both the X axis and Y axis. I need this to be visual to see the X and Y intercepts; that data goes into further calculations for enzyme kinetics. I apologize if this is too much information, but I want to be (hopefully :]) clear about what/why I need what I need.

Thanks for the help!