You're appending the line to your bars
variable, which is the selection for the bars. You need to append the line to the SVG:
svg.append("line")
.style("stroke", "black")
.attr("x1", 0)
.attr("y1", y(0.07))
.attr("x2", width)
.attr("y2", y(0.07));
This also sets the coordinates correctly. Remember that you have no data bound to the line, so function(d) { ... }
won't work.
Complete demo here. I've also deleted a bunch of unnecessary and broken code.