This question is building on the [solution given here](https://tex.stackexchange.com/questions/328713/add-annotations-to-histogram). I wanted to add a thick line to the enteries that are "0" so that the histogram would look like this:

[![enter image description here](https://i.stack.imgur.com/E4KIq.png)](https://i.stack.imgur.com/E4KIq.png)

Here is the code:

```
\documentclass[border=3mm,
               tikz,
               preview
               ]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}
%---------------------------------------------------------------%
\begin{tikzpicture}
  \begin{axis}[
    ymin=0, ymax=10,
    xmin=-0.5, xmax=8,
    xtick={0, ..., 7},
    ytick={0, ..., 9},
    axis x line=bottom,
    axis y line=left,
    area style,
  ]
    \addplot+[ybar interval] plot coordinates {
      (-0.50, 0) (0.5, 9) (1.5, 7) (2.5, 0) (3.5, 2) (4.5, 6) (5.5, 0)
    };
    \path
      \foreach[count=\i from 0] \v in {0, 9, 5, 0, 2, 6, 0} {
        (\i, \v) node[below] {\v}
      }
      (axis description cs:1, 0) node[above left, align=center] {grey\\level}
    ;
  \end{axis}
\end{tikzpicture}
\end{document}

```