Color gradient for bars in pgfplots

Question

I want that each bar in my diagram has a color gradient from black to white (from left to the right). However, for the following code, the gradient exists for the entire diagram such that the left bar is black, the middle one is gray, and the right one is white.

\documentclass{report}

\usepackage{pgfplots}


\begin{document}

\begin{tikzpicture} 
    \begin{axis}[ybar] 
        \addplot[left color=black,right color=white] coordinates {(1,2) (2,3) (3,4)}; 
    \end{axis} 
\end{tikzpicture}

\end{document}

Answer 1

One workaround is be to use error bars for the shading. This works because each error bar uses a separate path, so the shading applies to the bars individually:

\documentclass{report}

\usepackage{pgfplots}


\begin{document}

\begin{tikzpicture} 
    \begin{axis}[ybar, ymin=0] 
        \addplot[
            draw=none, fill=none,
            error bars/y dir=minus,
            error bars/y fixed relative=1,
            error bars/draw error bar/.code 2 args={
                \fill[xshift=-6pt, left color=black, right color=white, draw=black] ##1 ++(12pt,0pt) rectangle ##2;}
         ] coordinates {(1,2) (2,3) (3,4)}; 
    \end{axis} 
\end{tikzpicture}

\end{document}

Answer 2

ATTENTION: this answer refers to something which is not part of any stable release at the time of this writing.

---

Recently, I added switches to the bar plot handler of the PGF developer version. It allows to inject custom code at begin bar and at end bar.

This allows to write

\documentclass{standalone}

\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture} 
    \begin{axis}[ybar]
        \addplot[left color=black,right color=white,
            at begin bar={%
                \begin{scope}%
                \shade\pgfextra
            },
            at end bar={%
                \endpgfextra;
                \end{scope}%
            },
        ]
            coordinates {(1,2) (2,3) (3,4)}; 
    \end{axis} 
\end{tikzpicture}

\end{document}

Eventually, there will be styles to simplify such constructs.