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# Unusual Titration Curve Explanation

From Costanzo’s Physiology book in Acid-Base Physiology chapter, this titration curve of a weak acid ($$\ce{HA}$$), in the context of discussing buffers is provided:

What confused me is that in this diagram, the $$\mathrm{pH}$$ is on the $$x$$-axis rather than the $$y$$-axis. Also, why does the peek of the curve has ($$\ce{HA}$$) only and the trough has ($$\ce{A-}$$) only?