\starttext
\startMPpage
draw (0,0)--(10,10)--(15,5)--(20,10);
draw (0,0)--(0,10)--(10,10)--(10,5)--(15,5)--(15,10)--(20,10) withcolor blue;
\stopMPpage

\startMPpage
 draw fullcircle scaled 2cm;
 dotlabel.bot(textext("(0,0)"),origin);
\stopMPpage

\startMPpage
 draw fullcircle scaled 2cm;
 drawarrow (0,0)--(1cm,0);
 label.bot(textext("$r$"),(5mm,0));
\stopMPpage
\stoptext