You cannot merge graphs or even edit the waterfall graph to do what you want, however you can have multiple scatter and line plots in a 3D XYZ graph so all you have to do is set up the data correctly to have it display on the "left wall". For the 3D XYZ plot, the wall will be either the YZ plane - in which case X values would be constant at the minimum X - or in the XZ plane - in which case Y values would be constant at the maximum Y.