function setClosest (o) {
      o.distance = distance;
      o.distanceX = distanceX;
      o.distanceY = distanceY;
      o.seriesIndex = i;
      o.dataIndex = j;
      o.x = x;
      o.y = y;
      check = true;
    }
Retrieves the nearest data point from the mouse cursor. If it's within
a certain range, draw a point on the overlay canvas and display the x and y
value of the data.
@param {Object} mouse - Object that holds the relative x and y coordinates of the cursor.