Quero adicionar sinais de grau aos números no eixo x, conforme mostrado nesta imagem. Como posso fazer isso?
\documentclass[tikz,border=5pt]{standalone}
\usepackage{pgfplots}
\usepackage{filecontents}
\begin{filecontents*}{Fre.dat}
0.0000000e+00 9.7322390e+00 1.3985949e+01 1.5525350e+01
2.5000000e+00 9.7404206e+00 1.4001132e+01 1.5546305e+01
5.0000000e+00 9.7651029e+00 1.4046619e+01 1.5609681e+01
7.5000000e+00 9.8067018e+00 1.4122249e+01 1.5717003e+01
1.0000000e+01 9.8659247e+00 1.4227833e+01 1.5870771e+01
1.2500000e+01 9.9437917e+00 1.4363269e+01 1.6074399e+01
1.5000000e+01 1.0041667e+01 1.4528713e+01 1.6332129e+01
1.7500000e+01 1.0161304e+01 1.4724777e+01 1.6648943e+01
2.0000000e+01 1.0304897e+01 1.4952727e+01 1.7030511e+01
2.2500000e+01 1.0475157e+01 1.5214662e+01 1.7483207e+01
2.5000000e+01 1.0675401e+01 1.5513636e+01 1.8014219e+01
2.7500000e+01 1.0909671e+01 1.5853762e+01 1.8631749e+01
3.0000000e+01 1.1182876e+01 1.6240296e+01 1.9345313e+01
3.2500000e+01 1.1500982e+01 1.6679755e+01 2.0166108e+01
3.5000000e+01 1.1871246e+01 1.7180085e+01 2.1107451e+01
3.7500000e+01 1.2302522e+01 1.7750913e+01 2.2185310e+01
4.0000000e+01 1.2805653e+01 1.8403905e+01 2.3418947e+01
4.2500000e+01 1.3393986e+01 1.9153253e+01 2.4831704e+01
4.5000000e+01 1.4084049e+01 2.0016348e+01 2.6045676e+01
4.7500000e+01 1.4896463e+01 2.1014686e+01 2.7132868e+01
5.0000000e+01 1.5857171e+01 2.2175104e+01 2.8395420e+01
5.2500000e+01 1.6999148e+01 2.3531505e+01 2.9867785e+01
5.5000000e+01 1.8364789e+01 2.5127277e+01 3.1594083e+01
5.7500000e+01 2.0009352e+01 2.7018776e+01 3.3631631e+01
6.0000000e+01 2.2006006e+01 2.9280464e+01 3.6056144e+01
\end{filecontents*}
\begin{document}
\begin{tikzpicture}
\begin{axis}
\addplot file [x index=0,y index=1] {Fre.dat};
\end{axis}
\end{tikzpicture}
\end{document}
Responder1
Adicione xticklabel={$\pgfmathprintnumber{\tick}^\circ$}às opções do axis.
Aqui \tickestá uma macro que contém o número do tick atual, enquanto \pgfmathprintnumberimprime esse número (para que você não obtenha, por exemplo, 20.00000000001).
Talvez seja útil observar que xticklabelusado aqui não é o mesmo que xticklabels. O último permite especificar uma lista separada por vírgulas de ticklabels personalizados, enquanto xticklabel={..}permite especificar um "formato", como mostrado acima, onde o valor numérico do tick pode ser usado.
\documentclass[tikz,border=5pt]{standalone}
\usepackage{pgfplots}
\usepackage{filecontents}
\begin{filecontents*}{Fre.dat}
0.0000000e+00 9.7322390e+00 1.3985949e+01 1.5525350e+01
2.5000000e+00 9.7404206e+00 1.4001132e+01 1.5546305e+01
5.0000000e+00 9.7651029e+00 1.4046619e+01 1.5609681e+01
7.5000000e+00 9.8067018e+00 1.4122249e+01 1.5717003e+01
1.0000000e+01 9.8659247e+00 1.4227833e+01 1.5870771e+01
1.2500000e+01 9.9437917e+00 1.4363269e+01 1.6074399e+01
1.5000000e+01 1.0041667e+01 1.4528713e+01 1.6332129e+01
1.7500000e+01 1.0161304e+01 1.4724777e+01 1.6648943e+01
2.0000000e+01 1.0304897e+01 1.4952727e+01 1.7030511e+01
2.2500000e+01 1.0475157e+01 1.5214662e+01 1.7483207e+01
2.5000000e+01 1.0675401e+01 1.5513636e+01 1.8014219e+01
2.7500000e+01 1.0909671e+01 1.5853762e+01 1.8631749e+01
3.0000000e+01 1.1182876e+01 1.6240296e+01 1.9345313e+01
3.2500000e+01 1.1500982e+01 1.6679755e+01 2.0166108e+01
3.5000000e+01 1.1871246e+01 1.7180085e+01 2.1107451e+01
3.7500000e+01 1.2302522e+01 1.7750913e+01 2.2185310e+01
4.0000000e+01 1.2805653e+01 1.8403905e+01 2.3418947e+01
4.2500000e+01 1.3393986e+01 1.9153253e+01 2.4831704e+01
4.5000000e+01 1.4084049e+01 2.0016348e+01 2.6045676e+01
4.7500000e+01 1.4896463e+01 2.1014686e+01 2.7132868e+01
5.0000000e+01 1.5857171e+01 2.2175104e+01 2.8395420e+01
5.2500000e+01 1.6999148e+01 2.3531505e+01 2.9867785e+01
5.5000000e+01 1.8364789e+01 2.5127277e+01 3.1594083e+01
5.7500000e+01 2.0009352e+01 2.7018776e+01 3.3631631e+01
6.0000000e+01 2.2006006e+01 2.9280464e+01 3.6056144e+01
\end{filecontents*}
\begin{document}
\begin{tikzpicture}
\begin{axis}[xticklabel={\pgfmathprintnumber{\tick}$^\circ$}]
\addplot file [x index=0,y index=1] {Fre.dat};
\end{axis}
\end{tikzpicture}
\end{document}