Bar graph added.

node_modules/vega-functions/src/functions/log.js

@@ -0,0 +1,20 @@
+function log(df, method, args) {
+  try {
+    df[method].apply(df, ['EXPRESSION'].concat([].slice.call(args)));
+  } catch (err) {
+    df.warn(err);
+  }
+  return args[args.length-1];
+}
+
+export function warn() {
+  return log(this.context.dataflow, 'warn', arguments);
+}
+
+export function info() {
+  return log(this.context.dataflow, 'info', arguments);
+}
+
+export function debug() {
+  return log(this.context.dataflow, 'debug', arguments);
+}