In the Acid-base map -shown below- each line denoted as an isohydric line shows the pairs of $ PCO_{2}$ & $ [HCO_{3}^{-}] $ that give a certain $ pH $. So each line connects the values of $ PCO_{2}$ & $ [HCO_{3}^{-}] $ that give a value of $ pH $ that is constant across that line..
The 7.4 line shows the isohydric line of physiological pH,the central ellipse shows the normal range.
I understand the Henderson–Hasselbalch equation:
$\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\log _{10}\left(\frac{[\mathrm{Base}]}{[\mathrm{Acid}]}\right)$
What I would like to know is how this map established either experimentally or mathematically or both preferably.