scales.js

// this = marker under mouse 

const matrix = this.getScreenCTM()
  .translate(+this.getAttribute('cx'), +this.getAttribute('cy'));

const graphDiv = document.getElementById(graphOptions.chart.id).getBoundingClientRect();

let tooltTipLeftpx = matrix.e - graphDiv.left;
if (tooltTipLeftpx > (graphOptions.chart.width - (graphOptions.chart.padding * 1.5))) {
  tooltTipLeftpx -= graphOptions.chart.padding * 2;
}

tooltip.style({
  'left': `${tooltTipLeftpx}px`,
  'top': `${matrix.f - graphDiv.top}px`
});

Note: I am using a div outside the svg as a tooltip.