There's a way to do it that isn't pretty, you'll have to add in a phantom series with two data points.

See http://www.eng-tips.com/viewthread.cfm?qid=83015

You can set XY scales proportionally without macros, but using additional chart series consisting just of two data points to plot a diagonal line with equal tangents. The tangent length shall be dx=max(x)-min(x) or dy=max(y)-min(y) whichever is larger. Based on tangent length Excel will automatically set equal (or almost equal) limits to X and Y axes.

Step 1: Plot area shall be square (you can do it manually or in VBA (see IRstuff postage)
ActiveChart.PlotArea.Select
Selection.Width = 400
Selection.Height = 400

Step 2: Reserve 2x2 range for new series. Using IF function enter formulas for xo and yo

if dx >= dy

xo(1)=min(x) yo(1)=(max(y)+min(y)-dx)/2

xo(2)=max(x) yo(2)=(max(y)+min(y)+dx)/2

if dx < dy

xo(1)=(max(x)+min(x)-dy)/2 yo(1)=min(y)

xo(2)=(max(x)+min(x)+dy)/2 yo(2)=max(y)

Step 3: Add new series to the chart. Set line and marker width to none to make chart invisible.

The user providing the answer has a link to the SectProp software.