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