Hi Rustycar,
I don't believe there is a way to break an axis--I assume you mean a discontinuity--with Excel charts. However, you can make it appear to be a single chart with broken axis by actually overlaying two charts that have exactly the same x-axis scale, then setting the y-axis scales for the two charts separately to provide the discontinuity. Remove the x-axis from the chart with the higher y-axis values and move it up above the other charts, and bring it forward so it is on top of the other. By removing the background fill from the topmost chart the bottom chart can show through. You will also need to remove the border from the topmost chart.
I hope this is enough to get you going.