src/dygraph-interaction-model.js
function(x, left, right) {
  if (x < left) {
    return left - x;
  } else if (x > right) {
    return x - right;
  } else {
    return 0;
  }
}
function(x, left, right) {
  if (x < left) {
    return left - x;
  } else if (x > right) {
    return x - right;
  } else {
    return 0;
  }
}
Determine the distance from x to [left, right].