Question: Please help me to make IC50 graph with data as follow 

```
S(mM)   Activity (%) 
0.00080 99 %
0.00800 91 %
0.08000 89 %
0.40000 89 %
0.80000 79 %
1.60000 61 %
4.00000 39 %
8.00000 25 %
80.00000    4 %

```

and also IC50 value appear at 50% inhibition on the graph with curve fit line like EC50. 

I give sample (modefied by me) from web site <http://bpsbioscience.com/poly-adp-ribose-polymerase/inhibitors/xav-939-27100>
![enter image description here](https://i.stack.imgur.com/TTOwh.jpg)

Answer: 
As stated in the comments to @Habi's answer, `pgfplots` cannot perform a non-linear regression of your data. That's why you have to rely on an external tool (I used a quick `fit` routine in `gnuplot`). 

Once you obtained the fitted parameter(s), you can use `pgfplots` to draw a plot of the fitted curve/function, like so:

```
\documentclass{standalone}
\usepackage{pgfplots}
\usepackage{siunitx}
\DeclareSIUnit\Molar{\textsc{m}} % http://tex.stackexchange.com/a/27618/828
\pgfplotsset{compat=1.6}

\begin{document}
\begin{tikzpicture}
    \begin{semilogxaxis}[%
        xlabel={S / \si{\milli\Molar}},%
        ylabel={\% Activity},%
        legend pos=south west,%
        legend style={cells={anchor=west}}%
        ]
    \addplot [blue,mark=*,only marks] coordinates {
            ( 0.00080, 99)
            ( 0.00800, 91)
            ( 0.08000, 89)
            ( 0.40000, 89)
            ( 0.80000, 79)
            ( 1.60000, 61)
            ( 4.00000, 39)
            ( 8.00000, 25)
            (80.00000, 4)
            };
    \addlegendentry{data};
    % add plot of fitted curve with IC50=2.62645 mM (obtained via gnuplot)
    % %Activity = max * S / (IC50 + S) + min
    \addplot[red,smooth,domain=1e-3:100] { 100 * 2.63645 / (2.63645 + x) };
    \addlegendentry{sigmoidal fit}
    \end{semilogxaxis}
\end{tikzpicture}
\end{document}

```

![sample image](https://i.stack.imgur.com/n6w7f.png)
I suppose that's not what you want, is it?

```
\documentclass{article}
\usepackage{pgf} 
\usepackage{pgfplots}
\usepackage[active,tightpage]{preview}
\usepackage{siunitx}
\DeclareSIUnit\Molar{\textsc{m}} % http://tex.stackexchange.com/a/27618/828
\PreviewEnvironment{tikzpicture}

\begin{document}
\begin{tikzpicture}
    \begin{semilogxaxis}[%
        xlabel={S [\si{\milli\Molar}]},%
        ylabel=Activity%
        ]
    \addplot [smooth,blue,mark=*] coordinates {
            ( 0.00080, 99)
            ( 0.00800, 91)
            ( 0.08000, 89)
            ( 0.40000, 89)
            ( 0.80000, 79)
            ( 1.60000, 61)
            ( 4.00000, 39)
            ( 8.00000, 25)
            (80.00000, 4)
            };
    \end{semilogxaxis}
\end{tikzpicture}

\end{document}

```

which ends up to look like this: ![image from code above](https://i.stack.imgur.com/O8DpC.png)

**update**: code changed to reflect the example posted.