Dear Wolfgang,

Now, I understand why the error messages “\texfont0 is undefined”, “ you used the started character from an undefined font family.” are appeared.

Thank you for your clear explanation.

Best regards,

Dalyoung


3. removing \starttext, \stoptext, and functionstyle=“math”, it worked. However the caption 
“x=[…];  y=[….]” is overlapped.

ConTeXt doesn't preload fonts and only when no font was set untill \starttext latin modern is used as fallback font at this point.

When you omit \starttext no font is loaded and when you use math mode (which is the case when you use funtionstyle=math) the error message appears. Another fallback in this case it to load a monospaced font in horizontal or vertical boxes which is what you set in your example when you remove \starttext.