What do you call the line y=0 in a 2-dimensional plot? It's not the x-Axis, since i have the x-Axis below the plot. Is it called zero line?
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$\begingroup$In the spirit of Arthur's comment:
I believe what @kutschkem is after is to avoid confusion, as the auxillary line that helps the spectator of his plots to keep track of x-coordinates is not placed at its customary place. Whether it is the customary place or the ruler itself that is referred to as the x-axis is not something I would brush off as trivial. Whichever it is, what do you call the other?
To avoid confusion in such a scenario, you can simply refer to the line $y = 0$ exactly as that: $$\text{"the line}\;y = 0.\text{"}$$
I would not, however, refer to the line, in your case, $y = -0.25$ as the $x$-axis. That term is "too loaded". Again, if you need to reference that auxiliary line ruling the plane, refer to it as the line $y = -0.25$.
Customarily, the $x$ axis is defined as the line $y = 0$, and the $y$-axis as the line $x = 0$, but whatever the case, you are safe in referring to the line in question as $y = 0$.
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