hide bars representing '0' Fixes jupyter#524

see http://stackoverflow.com/questions/22378785/nvd3-charts-remove-0-value-bars

(c) Copyright IBM Corp. 2016

Author: peller

elements/urth-viz-chart/urth-viz-chart.css

@@ -1,2 +1,6 @@
 #chart1 .selected { fill: yellow !important; }
 #settings-section { display: none; }
+
+.nvd3 rect[height="1"] {
+  display: none !important;
+}