This example visualizes the classic Iris dataset, plotting sepal length against sepal width for three species of iris flowers. Each species is distinguished by both color (red, orange, blue) and symbol shape (triangle, cross, star), with a separate least-squares regression line overlaid for each group to show the linear relationship between the variables. The visualization uses D3 v4 with SVG rendering, employing `d3.scaleLinear` for axes, `d3.symbol` for the categorical point markers, and custom regression calculations to draw the trend lines.