This double line chart compares monthly passenger counts for United Airlines and American Airlines at San Francisco International Airport from January to June 2021. The visualization uses d3.nest to group data by airline, then draws two paths with d3.line, scaling the Month and APC values with d3.scaleLinear. Color is applied via d3.scaleOrdinal, with axes rendered using d3.axisBottom and d3.axisLeft. The data originates from the sfo_data.tsv file, and the SVG output uses an ordinal palette of red and blue.