Custom Spacing on PGF legend

Question

I'm trying to change the spacing of a single legend entry in a PGF bar plot. This is my MWE:

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
  \begin{tikzpicture}
    \begin{axis}[ybar]
        \addplot coordinates {(1,3) (2,5) (3,2)};
        \addlegendentry{(a)}
        \addplot coordinates {(1,2) (2,6) (3,1)};
        \addlegendentry{(b)}
        \addplot coordinates {(1,4) (2,4) (3,4)};
        \addlegendentry{(c)}
    \end{axis}
  \end{tikzpicture}
\end{document}

which generates the following figure

What I want is to create a space between (a) and (b) in the legend, while not creating a space between (b) and (c) (that is, I don't want to change the row sep). The ideia is to separate (a) from (b) and (c) in order to highlight that (a) is the result of a work and (b) and (c) are the result of another work in a presentation.

I tried to create a custom legend using \label and \ref, but this changes the bars (in the legend) to lines.

Answer 1

You can adjust the spacing of the legend entry by including text depth=2ex, anchor=mid in the \addlegendentry options:

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
  \begin{tikzpicture}
    \begin{axis}[ybar]
        \addplot coordinates {(1,3) (2,5) (3,2)};
        \addlegendentry[text depth=2ex, anchor=mid]{(a)}
        \addplot coordinates {(1,2) (2,6) (3,1)};
        \addlegendentry{(b)}
        \addplot coordinates {(1,4) (2,4) (3,4)};
        \addlegendentry{(c)}
    \end{axis}
  \end{tikzpicture}
\end{document}

Answer 2

Not the best solution, but you could add an empty legend entry with

\addlegendimage{empty legend}
\addlegendentry{}

The spacing can be increased by adding a zero width rule in the legend entry, e.g.

\addlegendentry{\rule{0pt}{3pt}}

Change 3pt to modify the spacing.

\documentclass{article}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
  \begin{tikzpicture}
    \begin{axis}[ybar]
        \addplot coordinates {(1,3) (2,5) (3,2)};
        \addlegendentry{(a)}
        \addlegendimage{empty legend}
        \addlegendentry{}
        \addplot coordinates {(1,2) (2,6) (3,1)};
        \addlegendentry{(b)}
        \addplot coordinates {(1,4) (2,4) (3,4)};
        \addlegendentry{(c)}
    \end{axis}
  \end{tikzpicture}
\end{document}