Very basic question about vectorial representation of an electromagnetic wave
The x and z directions in this graph are not spatial dimensions. 'x' is magnetic field and 'z' is electric field. 'y' is the only spatial dimension in the graph. So your point 'X' just represents a condition of a certain electric field for a different (stronger) EM wave (or DC field).

The graph is likely meant to represent the fields of a plane wave, because that's a relatively easy situation to analyze. In a plane wave travelling along the 'y' direction, the fields don't depend on location in the 'x' or 'z' directions. So if you meant your mark 'X' to represent a point in space offset from some reference 'x' and 'z', then the fields at that location are the same as for any location with the same 'y'.