Explicitly set axis char to '.'

lib/bandwidth-chart.js

@@ -42,7 +42,8 @@ module.exports = values => {
       height: 20,
       padding: 0,
       pointChar: '*',
-      negativePointChar: '.'
+      negativePointChar: '.',
+      axisChar: '.'
     });
   } else {
     return '';