I see you introduced the notion of 3-dimensionality to the discussion if you wish to retain the same label in a 3-D discussion, something like '*where the label Coordinate X is understood as Distance in the x-direction' is needed. If you want to retain the label as ' Coordinate X' I suggest that you define it clearly with some statement such as '*where the label Coordinate X is understood as Distance'. The x-axis would be far better labelled as “Distance”, which is what you say it is. An “actual point” (to use your own term) of “ Coordinate X” is really an abscissa with a particular value (of Distance) taken from the permissible range of (0<=x<=0.6 in this case) and should not be confused with the x-axis itself (which notionally extends to infinity in either direction).

It is confusing and redundant to label the x-axis as “coordinate X” because x is already understood by convention to be a coordinate. Corresponding values of y are shown in red. the permissible values of Distance) are limited to the range 0<=x<=0.6. The permissible values of the abscissae (i.e. This labelling of the x-axis is confusing and inappropriate, for the reasons below. Your graph shows an example of how the y- and x- coordinates are related when the ordinates are Values (density, intensity, etc) and the abscissae are something labelled ' X Coordinate'. An ordered pair consists of two terms – the abscissa (horizontal, usually x) and the ordinate (vertical, usually y) – which define the location of a point in two-dimensional rectangular space. Usually these are the horizontal and vertical coordinates of a point in a two-dimensional rectangular Cartesian coordinate system. In mathematics, the abscissa (plural abscissae or abscissæ or abscissas) and the ordinate are respectively the first and second coordinates of a point in a coordinate system:Ībscissa x-axis (horizontal) coordinate ordinate y-axis (vertical) coordinate The abscissa refers to the horizontal (x) axis and the ordinate refers to the vertical (y) axis of a standard two-dimensional graph. Wikipedia gives relevant background information: When referring to ' x-axis', one can understand the indefinite axis (no start and no end, since its an axis in space), so one might understand that they can attribute values 0 and 0.6 wherever on the axis and talk about the interval between 0 and 0.6 as if it exists wherever in space, wherever they want the origin of their x axis to be (and not where I set it to be) When referring to 'Coordinate x' or ' x-coordinate', one can understand 'talking about an actual point of coordinate x' and not 'along the axis where the x values are situated', since one can argue that the 'coordinate' is a fixed position in space and not an axis label I've searched and found all expressions somewhat common, with the 3rd example being the most common (although not in scientific context, more about math examples)

Variation of Value with the x-coordinate.

When referring to it, how is it correct to say? In a scientific paper, the label of the abscissa (the horizontal axis) is 'Coordinate x' and it represents the variation in space along the x axis between two points (along the thickness of a sample, where point 0.0 represents one end and point 0.6 represents the other end) of a placeholder function named 'Value' (it can be 'Density', 'Conductivity', 'Intensity' etc.) - see attached image.

Abscissa etymology