This chart shows a line graph comparing the monthly average passenger counts for United and American Airlines at SFO from January through June 2021. The visualization uses D3.js v4 to render an SVG, reading data from a TSV file. The code groups the data by airline using a `d3.nest` key function, then draws a separate colored path for each carrier with `d3.line`, scaled by a linear `d3.scaleLinear` for both the month (x-axis) and passenger count (y-axis). The final chart color palette maps airline names to blue and light blue.