What other graphical methods can be used to represent weathering?
First of all, you probably used XRF, and not XRD. XRF is a method used to obtain chemical compositions. XRD is used to obtain crystal structures.

A ternary diagram would be indeed very useful in this case. Another thing you might want to try is ratios: a plot of Al vs. K/Ca or something similar. Potassium and calcium likely behave differently in different weathering regimes, so it could be useful to show it.