Two scatterplots compare daily PM2.5 concentrations over time in 2018, with one chart treating the date as a categorical axis and the other as a continuous time axis. Both use the `d3.scaleLinear` for the x-axis and `d3.scalePoint` for the y-axis, rendered as SVG circles via `d3.select` and `d3.append`. The data comes from `2018.csv`, loaded with `d3.csv`. Axis ticks are generated with `d3.axisLeft` and `d3.axisBottom`, with the PM2.5 axis labeled explicitly.