title: "No data available" in Zeppelin charts
body: <p>I'm having problems creating visualizations with Zeppelin. I've got a dataset with about 600 million records. It's stored in an HDFS cluster and I'm able to load as a Spark dataframe:</p>
  
  <pre><code>%spark.pyspark
  input_hdfs_path = u'hdfs://cluster-master:9000/data/CDR_*.parquet'
  df = spark.read.format('parquet').load(input_hdfs_path)
  df.registerTempTable("df")
  </code></pre>
  
  <p>I'm interested in creating histograms on the length of the CDR (field <code>CDR_LENGTH</code>):</p>
  
  <pre><code>%sql
  select ROUND(CDR_LENGTH, -2) as duration, count(*) as count
  from df
  group by 1
  order by 1
  </code></pre>
  
  <p>I do get the appropriate results in the Table tab (with two columns, <code>duration</code> and <code>count</code>), but when going to the bar chart tab (or any other graphic tab), it simply says "No data available". Can you figure out what I'm doing wrong? Thanks</p>