It is important that all ion concentrations are expressed in equivalents (e.g. meq/L) before plotting.

With sum of the cation on the y-axis and sum of anions on the x-axis, points above the 1:1 line indicate carbonate weathering whereas points below indicate silicate weathering. 

How to draw the 1:1 line depends on which software you use. 

You can use excel for the same, make sure concentrations are expressed by same unit.

Thanks Marcus and Imran