Hi,

I'd like to offset the tip of the arrow at (a1) in the following example by 0.5mm in y- and x-direction. (This is an example from the chemfig manual.)

```
[xshift=-0.5mm, yshift=0.5mm]
```
does shift (db), however (a1) doesn't and I really don't get why.


```
\documentclass[a4paper,10pt]{article}  
  
\usepackage{chemfig}  
  
\begin{document}  
  
\schemestart  
\chemfig{@{a1}=_[@{db}::30]-[::-60]\charge{90:2pt=\|}{X}}  
\arrow{<->}  
\chemfig{\chemabove{\vphantom{X}}{\ominus}-[::30]=_[::-60]  
\chemabove{X}{\scriptstyle\oplus}}  
\schemestop  
\chemmove{\draw([xshift=-0.5mm, yshift=0.5mm]db).. controls +(120:5mm) and +(130:5mm).. ([xshift=-0.5mm, yshift=0.5mm]a1);}  
  
\end{document}
```


I'd really appreciate some explanation why and how I could do it.

Thank You!