You have a total of 6 variables on the y-axes, but only 3 variables on the x-axis. The way Mathcad resolves that is y1 vs x1, y2 vs x2, y3 vs x3, y4 vs x3, y5 vs x3, y6 vs x3. If you put x1,x2,x3,x1,x2,x3 on the x-axis you will get y1 vs x1, y2 vs x2, y3 vs x3, y4 vs x1, y5 vs x2, y6 vs x3, which is what you want. When you have fixed that you will still see 6 curves though, because the lower and upper limits of the two axes are not identical. In this case the lower limit is 0, so you can make the graphs overlap exactly by putting 100*ksi/MPa in the upper limit placeholder on the MPa axis. Also set the number of grids to be the same for the two axes. The tick marks are then at non-integer numbers, but you can't have everything.
