Rule disappears

[Patrick X Cao]

I tried to create a rule by:

  d3.select("body").append("div")
      .attr("class", "rule")
      .call(context.rule());

Even though I can see the values of where my mouse is in the charts, I just can't see the rule line. It's like it becomes invisible.

How can I change the color of that line? Or what did I do wrong?

[Patrick X Cao]

Added a simple CSS file with:
.line {
background: #000;
z-index: 2;
}
The rule appeared.