d3-data-formatting

name	d3-data-formatting
description	Loading CSV data, formatting currency/large numbers, and implementing tooltips in D3.

Data Processing and Tooltips in D3

Efficient data visualization requires robust parsing and clear numerical presentation.

Loading Multiple CSVs

Use Promise.all with d3.csv to load multiple datasets before initializing the visualization.

Promise.all([
    d3.csv("data/stock-descriptions.csv"),
    d3.csv("data/indiv-stock/AAPL.csv")
]).then(([descriptions, aaplData]) => {
    // Process and merge data
});

Number Formatting

For market capitalization, use d3.format or custom logic to convert large numbers into readable strings (e.g., Trillions 'T', Billions 'B').

function formatMarketCap(d) {
    if (!d || d === "") return "N/A";
    const val = parseFloat(d);
    if (val >= 1e12) return (val / 1e12).toFixed(2) + "T";
    if (val >= 1e9) return (val / 1e9).toFixed(2) + "B";
    if (val >= 1e6) return (val / 1e6).toFixed(2) + "M";
    return val.toLocaleString();
}

Tooltips

Create a hidden div and update its position and content on mouse events.

const tooltip = d3.select("body").append("div")
    .attr("class", "tooltip")
    .style("opacity", 0);

nodes.on("mouseover", (event, d) => {
    tooltip.transition().duration(200).style("opacity", .9);
    tooltip.html(`Ticker: ${d.ticker}<br/>Sector: ${d.sector}`)
        .style("left", (event.pageX + 10) + "px")
        .style("top", (event.pageY - 28) + "px");
})
.on("mouseout", () => {
    tooltip.transition().duration(500).style("opacity", 0);
});