Here's the code. This demonstrates that linkcatcher does not catch links sent from within an SVG diagram. Is there a work-around (other than using foreignObject and buttons) ?
\define linkaction()<$action-setfield $tiddler=<<currentTiddler>> $field="curitem" $value="u"/>
<$linkcatcher actions=<<linkaction>> >
<svg stroke="black" fill="white" fill-opacity="0" width="100" height="100">
<a href="#ggg"><rect x=1 y=1 width=20 height=20 /></a>
</svg>
</$linkcatcher>
<$linkcatcher actions=<<linkaction>> >
[[ggg]]
</$linkcatcher>
{{!!curitem}}
<$button set="!!curitem" setTo="X">reset</$button>