Angle of a triangle within unit circle

Question

I am trying now for a while to add an angle to my triangle, but unfortunately I receive the following error: "Package PGF Math Error: Unknown function `axis' (in 'axis cs')." My Latex code looks like the following:

\documentclass[12pt]{article}

\usepackage[german]{babel}

\usepackage{ucs}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fancyhdr}
\usepackage{blindtext}
\usepackage{listofsymbols}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{xcolor}
\usepackage{longtable}
\usepackage{stmaryrd}
\usepackage{graphicx}
\usepackage{pgfplots}
\usepackage{tkz-euclide}
\usepackage{calc}    
\begin{center}
    \begin{tikzpicture}
        \begin{axis}[
            axis lines=center,
            xtick={-1, 1},
            ytick={-1, 1},
            xlabel={$x$},
            ylabel={$y$},
            xlabel style={below right},
            ylabel style={above left},
            xmin=-1.5,
            xmax=1.5,
            ymin=-1.5,
            ymax=1.5,
            axis equal image]
            
            \draw[color=teal] (axis cs:0,0) -- (axis cs:0.71,0) -- (axis cs:0.71,0.71) -- cycle;
            \draw[fill=lightgray, thick] (axis cs: 0,0) -- (axis cs: 0,0) arc (axis cs: 0.71,0) node at (axis cs: 0.71,0.71) {$\alpha$} -- cycle;
            
            \draw(axis cs: 0,0) circle [radius=100];
        \end{axis}
    \end{tikzpicture}
    \end{center}~\\

The error appears while trying to draw a lightgray arc. Thanks for helping me out, JackboyPlay.

Answer 1

• Add \pgfplotsset{compat=1.18}, then axis cs is the default.
• arc has a different syntax, see https://tex.stackexchange.com/a/175027

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        axis lines=center,
        xtick={-1, 1},
        ytick={-1, 1},
        xlabel={$x$},
        ylabel={$y$},
        xlabel style={below right},
        ylabel style={above left},
        xmin=-1.5,
        xmax=1.5,
        ymin=-1.5,
        ymax=1.5,
        axis equal image]
        
        \draw[color=teal] (0,0) -- (0.71,0) -- (0.71,0.71) -- cycle;
        \draw[fill=lightgray, thick] (0,0) -- (0.71,0) arc (0:45:0.71) -- cycle;
        \node at (22:.30)  {$\alpha$};% polar coordinate: 22 degrees, distance 0.30
        \draw(0,0) circle (1);
    \end{axis}
\end{tikzpicture}
\end{document}