Hepl to make a graph with 3 positive variables

Not sure that it's the most esthetic way of doing, but this does *something*.

I am not an expert, but I think you have to (and even if you don't have to, at least, it works this way) use the "fundamental" variables that your stuff are built on to define everything (here, pe and p1, not x and alt).

You also have to define them as range variable for plotting (it might assume range variable on its own, but I wouldn't let the software make that decision for me). In you particular case, you don't have much choise, because the function you want to plot are functions of more than one variable, so I doubt that Mathcad can decide on its own which one you want to do you plot against.