Static position on container messes up which plot point is hovered

[nchen63]

Hovering over plot points can hover over the wrong point if there is a static container somewhere in the chart's ancestors.

The fix is to check if the ancestor statically positioned in getCumulativeTransform(). Something functionally equivalent to:

        var styles = window.getComputedStyle(elem);
        if (styles.getPropertyValue("position") == "static") {
            return transform;
        }