I would like to add a legend header to one of my pgfplot. So far I get that

\documentclass{standalone}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
ymin=0,
ymax=1,
enlargelimits=false,
legend pos=north west,
legend style={area legend},
legend columns=-1
]
[const plot,fill=blue,draw=black]
coordinates
{(0,0.1)    (0.1,0.15)  (0.2,0.5)   (0.3,0.62)
(0.4,0.56) (0.5,0.58)  (0.6,0.65)  (0.7,0.6)
(0.8,0.58) (0.9,0.55)  (1,0.52)}
\closedcycle;
[const plot,fill=red,draw=black]
coordinates
{(0,0.1)    (0.1,0.15)  (0.2,0.5)   (0.3,0.62)
(0.4,0.56) (0.5,0.58)  (0.6,0.65)  (0.7,0.6)
(0.8,0.58) (0.9,0.55)  (1,0.52)}
\closedcycle;
\end{axis}
\end{tikzpicture}
\end{document}


which gives me that

I would like to have the second row to be left aligned with respect to the first one. I do not want to fix the number of row because there may be more histograms to come and I would like to have them horizonthally aligned (that's why I use legend columns=-1).

I read here that pgfplots legend is nothing else than tikz matrix so maybe there is a way to x-shift the second row (as I already did with the yshift=10pt command).

-
So you want some sort of multicolumn matrix? This is unsupported by PGF's \matrix as far as I know. A patch has been proposed in tex.stackexchange.com/questions/1388/… - perhaps that might work here as well –  Christian Feuersänger Nov 10 '12 at 17:15
@ChristianFeuersänger: I saw this answer yesterday but I didn't get time to try it. I will edit my question if I can make it work. –  Xavier Garrido Nov 10 '12 at 18:43
@ChristianFeuersänger: I didn't manage to make it work but I find another way to do it (see the answer below) –  Xavier Garrido Nov 11 '12 at 10:22

Using legend style I find something a bit crappy but working. Here is a possible solution

\documentclass{standalone}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
ymin=0,
ymax=1,
enlargelimits=false,
legend pos=north west,
legend style={area legend,
/tikz/row 1 column 2/.append style={column sep=-2.3cm}},
legend columns=-1
]
[const plot,fill=blue,draw=black]
coordinates
{(0,0.1)    (0.1,0.15)  (0.2,0.5)   (0.3,0.62)
(0.4,0.56) (0.5,0.58)  (0.6,0.65)  (0.7,0.6)
(0.8,0.58) (0.9,0.55)  (1,0.52)}
\closedcycle;
[const plot,fill=red,draw=black]
coordinates
{(0,0.1)    (0.1,0.15)  (0.2,0.5)   (0.3,0.62)
(0.4,0.56) (0.5,0.58)  (0.6,0.65)  (0.7,0.6)
(0.8,0.58) (0.9,0.55)  (1,0.52)}
\closedcycle;

Of course, the column sep=-2.3cm really depends on the text width of the header which may vary from one plot to another.