Suggestion: support responsive size (mouse events)

[mikkokam]

By adding this, sparkline interactionLayer can support responsive (% - based) sizes (css expanding the sparkline to fit a div, for instance).

var realWidth;

interactionLayer.addEventListener("mouseover", event => {
    realWidth = svg.clientWidth;
});

interactionLayer.addEventListener("mousemove", event => {
    const mouseX = (event.offsetX * width) / realWidth;
...