This example demonstrates a categorical line chart using D3.js, visualizing population frequencies across U.S. states by age group. The chart renders as an SVG with grid lines extending from both axes, and uses a cardinal interpolation for smooth curved lines. Each age-group category is encoded with a distinct color from a categorical scale, and a tooltip (via d3-tip) displays the state and value on hover. The x-axis labels are rotated for readability, and the y-axis is labeled "Frequencies." The chart’s data comes from a JSON file listing population counts for various age brackets per state, with support for toggling gridlines and rotation.