No png file is generated.

[hifall]

My code is like:

const fs = require('fs');
const output = require('d3node-output');
const d3 = require('d3-node')().d3;
const d3nPie = require('d3node-piechart');

const csvString = fs.readFileSync('./data.csv').toString();
const data = d3.csvParse(csvString);
const options = {width: 200, height: 200};

output('./export', d3nPie({ data: data }), options);

After I run the above code, these files are generated: export.html and export.svg, but not export.png.

How can I generate png as well?

Thanks!