Hi Aditya,
Thank you for your help ; this also works with pgfplots but the body of the text must be with the same font. I always use the pagella font but it doesn't allow you to use \mathscript{} that's why I use this macro. Are there other free fonts that allow you to do this ?
Fabrice
\usemodule[pgfplots]
\pgfplotsset{compat=newest}
\usebodyfont[xitsbidi]
\define[1]\cscript{\start\m{{\mathscript{#1}}}\stop}
\startmidaligned
\define[1]\cscript{\start\m{{\mathscript{#1}}}\stop}
\starttikzpicture
\startaxis
\addplot {x};
\node[above left] at (2,2) {\cscript{C}};
\stopaxis
\stoptikzpicture
\stopmidaligned
\blank
\startmidaligned
\switchtobodyfont[9pt]
\starttikzpicture
\startaxis
\addplot {x};
\node[above left] at (2,2) {\cscript{C}};
\stopaxis
\stoptikzpicture
\stopmidaligned
\stoptext