PGFplot multiple histograms using CSV file

I have tried to draw a comparative graph using histogram and taking data from different CSV source files. I can draw the two histograms but the resulting graph is not what I want: I don't want the two histograms to be merged. My tex file:

\documentclass{article}
\usepackage{siunitx}
\usepackage{tikz}
\usepackage{pgfplots}

\usepgfplotslibrary{units}

\sisetup{
round-mode          = places,
round-precision     = 2,
}

\begin{document}

\begin{figure}[h!]
\begin{center}
\begin{tikzpicture}
\begin{axis}[
width=15cm, height=8cm,
grid=major,
grid style={dashed,gray!30},
xlabel=String length,
ylabel=Performance ops/ms,
legend style={at={(0.5,-0.2)},anchor=north},
xmode=log,
log ticks with fixed point,
xtick=data,
]
\end{axis}
\end{tikzpicture}
\caption{Performance Comparison Histogram}
\end{center}
\end{figure}

\end{document}


Here is the output I get:

What I want is aligned histograms like this example in pgfplot gallery :

The following code is used to do that:

symbolic x coords={tool8,tool9,tool10},


I am not sure if there is a way to do the same without explicitly defining the value of the coordinates in the tex file.

Note: The value of the length in both files is the same {1,2,4,...,64}, only the score changes.

You need to add the ybar option to the axis options, instead of to each plot individually. Note that the author of pgfplots recommends not to use the newest compat option, and also I think that loading the pgfplotslibrary units is redundant if you also load siunitx. Finally, pgfplots already loads tikz so no need for duplication there either :-)

\documentclass{article}
\usepackage{siunitx}
\usepackage{pgfplots}

\pgfplotsset{compat=1.12}

\sisetup{
round-mode          = places,
round-precision     = 2,
}

\begin{document}

\begin{figure}[h!]
\begin{center}
\begin{tikzpicture}
\begin{axis}[
width=15cm, height=8cm,
grid=major,
grid style={dashed,gray!30},
xlabel=String length,
ylabel=Performance ops/ms,
legend style={at={(0.5,-0.2)},anchor=north},
xmode=log,
log ticks with fixed point,
xtick=data,