Hi Smoore2,
Using graph builder you can create a mosaic plot with either proportionally sized or equally sized x categories.
If you use your X variable in the X drop zone, you will produce the typical mosaic plot with proportionally sized categories on the X
But, if you use your X variable in the "Group X" drop zone at the top, JMP will size the categories equally.
If you right click the category labels on the top, then select "X Group Edge" you can move the labels to the bottom (to most closely resemble the original mosaic plot).
I hope this helps!
Julian