"Horizontal", in this context, means from left to right (or from right to left).
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It is the x axis
The horizontal number line on a coordinate plane is called the x-axis, and the vertical number line on a coordinate plane is called the y-axis.
I believe you mean the x-axis, the horizontal axis on a coordinate plane
X-axis
In the coordinate plane, the x-axis is horizontal, and the y-axis is vertical.