Limiting domain in PGFPlots gives erroneous results

Question

I am plotting a simple straight line graph. Actually, I'm plotting a modulus, or absolute, function which I had no success with and using two linear plots to do it instead.

My code is the following:

\pgfkeys{/pgfplots/axis labels at tip/.style={
 xlabel style={at={(current axis.right of origin)}, yshift=1.5ex, anchor=north},
 ylabel style={at={(current axis.above origin)}, xshift=1.5ex, anchor=east}}
 }
\begin{minipage}[t]{0.42\columnwidth}
   \begin{tikzpicture}
     \begin{axis}[
       xmin=-2, xmax=3,
       ymin=-4, ymax=6,
       scale only axis=true,
       scale=0.4,
       domain=-2:3,
       axis lines=middle,
       samples=500,
       xtick=\empty,
       ytick=\empty,
       ylabel={$y$},
       xlabel={$x$},
       axis labels at tip,
      ]
  \end{axis}
      \addplot [maincolor,restrict y to domain={0:6} ] { 2*x - 2};
\end{tikzpicture}

\end{minipage}

And this is the erroneous result

What I've tried to do is cut the graph off at the x-axis. But this graph is cut off below the x-axis. Why is this?

EDIT:

Here is a compilable example with some of the environments I have the graph in:

\documentclass{article}
\usepackage{multicol}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{ninecolors}
\usepackage{environ}
\colorlet{maincolor}{violet5}
% Define boxes
\tikzstyle{mybox} = [draw=maincolor, fill=white, very thick,
rectangle, rounded corners, inner sep=10pt, inner ysep=10pt]
\tikzstyle{fancytitle} =[fill=maincolor, text=white, rounded corners, font=\bfseries]
\NewEnviron{chbox}[1]
{
    \begin{tikzpicture}
      \node [mybox] (box){%
      \minipage{0.3\textwidth}
      \fontsize{7pt}{8pt}\selectfont
      \BODY
      \par
      \endminipage
     };
 \node[fancytitle, right=10pt] at (box.north west) { #1 };
 \end{tikzpicture}
}
\pgfkeys{/pgfplots/axis labels at tip/.style={
     xlabel style={at={(current axis.right of origin)}, yshift=1.5ex,  anchor=north},
     ylabel style={at={(current axis.above origin)}, xshift=1.5ex,  anchor=east}}
 }
\begin{document}
\begin{multicols*}{3}

   \begin{chbox}{Example}
       \begin{minipage}[t]{0.42\columnwidth}
           \begin{tikzpicture}
              \begin{axis}[
                 xmin=-2, xmax=3,
                 ymin=-4, ymax=6,
                 scale only axis=true,
                 scale=0.4,
                 domain=-2:3,
                 axis lines=middle,
                 samples=500,
                 xtick=\empty,
                 ytick=\empty,
                 ylabel={$y$},
                 xlabel={$x$},
                 axis labels at tip,
               ]
               \addplot [maincolor,restrict y to domain={0:6} ] { 2*x - 2};
              \end{axis}
             \end{tikzpicture}
            \end{minipage}
           \end{chbox}  
          \end{multicols*}

        \end{document}

It turns out that it seems to be my box around the plot which causes the error. If I remove the box, it cuts off perfectly at the x-axis.

Problem is, I need the boxes.

With the box:

Without the box:

Answer 1

You can use the fit library to avoid nesting of tikzpictures which can easily cause alignment problems:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\usepackage{ninecolors}
\usepackage{environ}
\colorlet{maincolor}{violet5}
\tikzset{
    mybox/.style={
        draw=maincolor, very thick, rectangle, rounded corners, inner sep=10pt, inner ysep=10pt
    },
    fancytitle/.style={
        fill=maincolor, text=white, rounded corners, font=\bfseries
    },
    /pgfplots/axis labels at tip/.append style={
        xlabel style={
            at={(current axis.right of origin)}, yshift=-1.5ex, anchor=north
        },
        ylabel style={
            at={(current axis.above origin)}, xshift=-1.5ex, anchor=east
        }
    }
}
\NewEnviron{chbox}[1]{
    \begin{tikzpicture}[font={\fontsize{7pt}{8pt}\selectfont}]
        \BODY
        \node[mybox, fit={(current bounding box.north east) (current bounding box.south west)}] (box) {};
        \node[fancytitle, right=10pt] at (box.north west) {#1};
    \end{tikzpicture}
}
\begin{document}
\begin{chbox}{Example}
    \begin{axis}[
        xmin=-2, xmax=3,
        ymin=-4, ymax=6,
        scale only axis=true,
        scale=0.4,
        domain=-2:3,
        axis lines=middle,
        samples=500,
        xtick=\empty,
        ytick=\empty,
        ylabel={$y$},
        xlabel={$x$},
        axis labels at tip,
    ]
    \addplot[maincolor, restrict y to domain={0:6}] {2*x - 2};
    \end{axis}
\end{chbox}  


\end{document}

Answer 2

I found a workaround to this.

The problem seems to be from the pgfplot being embedded in two tikzpictures, one for the box and one for the actual plot. Perhaps something happens to the coordinates when this happens.

So I used savebox which typesets it once before the tikzpicture and then just dumps it in.

I would still appreciate if somebody could tell me why the coordinates become confused and if there is a more natural way of doing this within tikz and pgfplots, rather than using saveboxes.

My workaround:

\newsavebox\mybox
\begin{lrbox}{\mybox}
 \begin{tikzpicture}
   \begin{axis}[
     xmin=-2, xmax=3,
     ymin=-4, ymax=6,
     scale only axis=true,
     scale=0.4,
     domain=-2:3,
     axis lines=middle,
     samples=500,
     xtick=\empty,
     ytick=\empty,
     ylabel={$y$},
     xlabel={$x$},
     axis labels at tip,
    ]
    \addplot [maincolor,restrict y to domain={0:6} ] { 2*x - 2};
\end{axis}
\end{tikzpicture}
\end{lrbox}
\begin{chbox}{Example}
  \begin{minipage}[t]{0.42\columnwidth}
    \usebox\mybox
  \end{minipage}
\end{chbox}

Now I'll just readjust the axis labels.