feat: enable ppi setting on png cli export (vega#8854)

bin/args.js

@@ -51,6 +51,9 @@ To load data, you may need to set a base directory:
   if (type === 'vega') {
     args.boolean('p').alias('p', 'pretty').describe('p', 'Output human readable/pretty spec.');
   }
+  else if (type === 'png') {
+    args.number('ppi').describe('ppi', 'Resolution in ppi.');
+  }
 
   return args.help().version().argv;
 };

bin/render.js

@@ -33,6 +33,9 @@ module.exports = (type, callback, opt) => {
   // set output image scale factor
   const scale = arg.scale || undefined;
 
+  // Allows for other ppi settings than 72 for png files
+  const ppi = arg.ppi || 72;
+
   // use a seeded random number generator, if specified
   if (typeof arg.seed !== 'undefined') {
     if (Number.isNaN(arg.seed)) throw 'Illegal seed value: must be a valid number.';
@@ -55,7 +58,7 @@ module.exports = (type, callback, opt) => {
       renderer: 'none' // no primary renderer needed
     }).finalize(); // clear any timers, etc
 
-    return (type === 'svg' ? view.toSVG(scale) : view.toCanvas(scale, opt)).then(_ => callback(_, arg));
+    return (type === 'svg' ? view.toSVG(scale) : view.toCanvas(scale * ppi / 72, opt)).then(_ => callback(_, arg));
   }
 
   // read input from file or stdin

bin/vl2png

@@ -7,6 +7,6 @@ const render = require('./render');
 render('png', function(canvas, arg) {
   const file = arg._[1] || null;
   const out = file ? createWriteStream(file) : process.stdout;
-  const stream = canvas.createPNGStream();
+  const stream = canvas.createPNGStream({ resolution: arg.ppi || undefined });
   stream.on('data', chunk => { out.write(chunk); });
 });