To add horizontal lines on bar charts in d3.js, you can create a separate SVG element within the same SVG container as the bar chart itself. In this new SVG element, you can append lines using d3's append method.Specify the starting and ending points of the lines using the x1, y1, x2, and y2 attributes. You can also customize the appearance of the lines by setting attributes such as stroke, stroke-width, and stroke-dasharray.