Data conversion transforms a value from one data type into another. When D3 loads a CSV file, numeric values commonly arrive as strings, so counts must be converted before they drive a visualization. section 3.2.2
Convert counts while loading CSV data
Use d3.csv()'s row conversion callback to return only the fields needed by the visualization and convert count with the unary + operator: d3.csv("../data/data.csv", d => { return { technology: d.technology, count: +d.count }; }); The returned key-value pairs are the fields available after the dataset loads. If many columns must remain, format the complete dataset afterward instead. section 3.2.2
Figure 3.9 The third step of the D3 data workflow consists initially of formatting the data so that it’s ready to use when you start building your visualization.
Follow data conversion as the formatting step that prepares imported data for visualization. section 3.2.2
