Brushing inside transformed container #145

First off, hat 👒 off for this breath of fresh air! I've gone through at least five chart libraries of which none gets what svelteplot gets: You definitely want declarative charts in a reactive environment.

As for me, I'm putting charts into svelte-flow nodes, but it might as well be inside three.js planes: a grandfather container might have transformations. Drawing the charts goes spotless – the issue comes when its time for brushing.

Problem
When is used inside transformed containers (CSS transforms, SvelteFlow zoom/pan, Three.js planes), the brush rectangle draws at incorrect screen coordinates while the data selection remains accurate.

Root Cause
The clientToLayerCoordinates() function converts event.clientX/clientY using:

return [event.clientX - plotBodyRect.left, event.clientY - plotBodyRect.top];

This doesn't account for ancestor CSS transforms, causing coordinate misalignment proportional to the transform scale.

Proposed Solution
Add an optional coordinateTransform prop to the <Brush> component:

<Plot>
  <Brush 
    x="date" 
    coordinateTransform={myTransformFn}
    onbrush={handleBrush} 
  />
</Plot>

<script>
function myTransformFn(clientX, clientY, plotBody) {
  // Apply container-specific coordinate correction
  const corrected = containerToLocal(clientX, clientY);
  return [corrected.x, corrected.y];
}
</script>

** Implementation**
Modify clientToLayerCoordinates() to accept and use the transform function:

export function clientToLayerCoordinates(event, plotBody, transformFn = null) {
    if (!plotBody) return [0, 0];
    
    const plotBodyRect = plotBody.getBoundingClientRect();
    
    if (transformFn) {
        const [transformedX, transformedY] = transformFn(event.clientX, event.clientY, plotBody);
        return [transformedX - plotBodyRect.left, transformedY - plotBodyRect.top];
    }
    
    return [event.clientX - plotBodyRect.left, event.clientY - plotBodyRect.top];
}

Hmm... at this point I realise the clientToLayerCoordinates runs as soon as I move the mouse. It might make sense to make the incision in the Brush component...