• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

There seems to be no way to accurately prevent a label for a ReferenceLine from exceeding the bounds of the view box. The viewBox argument to the custom function always has a width: 0.

The following screen shot shows how my label renders when it's positioned toward the right bound of the box. If I knew the width of the box I could shift it left so that the whole label is visible.

Here's the markup for the actual label:

<g>
  <rect
    x={rectX}
    y={y}
    fill={markerFill}
    stroke={markerStroke}
    width={rectWidth}
    height={rectHeight}
    rx={3}
    ry={3}
  />
  <text
    x={rectX + rectWidth / 2}
    y={y + rectHeight / 2}
    dominantBaseline="middle"
    textAnchor="middle"
    fill={markerText}
  >
    {text}
  </text>
</g>

The problem is that I have no way to calculate the value of rectX since I do not know how wide the viewBox is.

What does the proposed API look like?

Pass the actual width of the view box to the label function.