I'm drawing a simple scatter plot with pgfplots, which also includes a legend as shown in the upper part of the picture next.

However, I find the horizontal spacing odd because the distance between the symbol and the corresponding description is the same as to the next description. I would like to have it like "faked" in the lower part of the picture.
A simple example is produced by the following code. Here, the descriptions are a,b,c,d.
\documentclass{scrartcl}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[legend columns=-1, legend pos=outer north east]
\addplot+ coordinates {(0,0) (0,1)};
\addplot+ coordinates {(1,0) (0,0)};
\addplot+ coordinates {(0,0) (1,1)};
\addplot+ coordinates {(1,1) (0,0)};
\legend{a,b,c,d};
\end{axis}
\end{tikzpicture}
\end{document}
If I understand the manual correctly, the legend is composed of a matrix. However, I could not figure out how to change the spacing accordingly. Your help is much appreciated ;)
