pgfplots function which touches the edges of the plot

Question

I need to plot the constant y = pi function with the plot touching both sides of the plot.

I have had a search through the pgfplots manual without much luck. I have tried \addplot{3.141...} which produces a small line spanning very little of the x axis, and manually plotting points for the range of my graph.

Answer 1

I see two ways to achieve what you want, depending on what you mean by "touching both sides of the plot".

You can add an extra y tick. Or, you can add a plot that is constant either by using coordinates or function. The coordinates one needs the xmin and xmax to be specified.

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[xlabel={$x$},
    ylabel={$\arctan(x)$},
    extra y ticks={0.785},
    extra y tick labels={$\pi/4$},
    extra y tick style={grid=major},
    minor y tick num=1,
    domain=-5:5,
    samples=100]
    \addplot+[no marks] function {atan(x)};
    \addplot+[no marks] function {pi/2};
    \addplot+[no marks] coordinates {(-5,pi) (5,pi)};
  \end{axis}
\end{tikzpicture}
\end{document}

Answer 2

You have either of the following two as an option:

\begin{axis}
  \begin{pgfplotsinterruptdatabb}
    \addplot[forget plot] coordinates {(-16000,3.14) (16000,3.14)};
  \end{pgfplotsinterruptdatabb}
\end{axis}

or

\begin{axis}
  \addplot[forget plot,update limits=false] coordinates {(-16000,3.14) (16000,3.14)};
\end{axis}

The first I would prefer. You can then easily do:

\newcommand\addplotpi[1][]{
  \begin{pgfplotsinterruptdatabb}
    \addplot[forget plot,#1] coordinates {(-16000,3.14) (16000,3.14)};
  \end{pgfplotsinterruptdatabb}
}

and simply use that in every axis environment you wish to utilize it in.

The forget plot allows you to add it in the beginning of an axis environment without disturbing the color-lists and legends.

The only thing you should worry about is the lower and upper limit of x. But just do it large enough so that it wont bother you! :)