Formula for $P\small CO_2$ Not really sure where I was supposed to ask this question. I have no background/knowledge in chemistry or math of this level. 
This is a follow-up question from my question over at StackOverflow (question) regarding the Henderson-Hasselbalch equation and how implement in a web environment that uses $x$/$y$ coordinates for plotting.
I think reading the StackOverflow question will most likely clarify what I'm asking for specifically.

The Davenport Diagram in question: http://imgur.com/Lo87Aib
Since the tool I use to plot it uses $X$ and $Y$ coordinates, would it be even possible to calculate the $P\small CO_2$ (red lines) value-based of the $HCO_3$ ($Y$ coordinate) and $pH$ ($X$ coordinate). I figure that it would need a custom-tailored formula (if it is even possible). $P\small CO_2$ is a constant in the Henderson-Hasselbalch equation. 
I searched for it a whole lot, I am not sure if I was even searching correctly since the jargon used on all those sites makes no sense to me.

Well, the formula seems to be
$$ pH = 6.1 + \log_{10} \left(\frac{HCO_3^{-}}{0.03 \times P_{co_2}}\right)$$
Hence
$$ P_{co_2}=\frac{HCO_3^{-}}{0.03} 10^{6.1-pH}  \approx  HCO_3^{-} \times 10^{7.623-pH}  $$
This seems to fit with your graph (eg: $HCO_3^{-}=16$, $pH=7.65$ $P_{co_2}=15.03$)
