Dear Henri Menke, I always use “btex etex” for labeling math. But “…..” is simpler and it works fine. Thank you. Best regards, Dalyoung

...

\startbuffer[delta] label.bot(btex ${\delta}$ etex, (1cm,0)); % %label.bot(btex $\delta$ etex, (3cm,0));

Why not just

label.bot("$\delta$", (3cm,0));