I tried the same:
SELECT total_bytes_read_rate_across_datanodes,total_bytes_written_rate_across_datanodes where serviceType=HDFS and category=SERVICE
And it worked out fine.
One thing I did not, though, is that in my chart information, I could see the HDFS service name listed in the legend like this:
Perhaps it is a clue that you have no HDFS Service Name there.
Perhaps try going to HDFS Status page in Cloudera Manager and then click on the Total Bytes Written Across DataNodes chart to see if it shows any data.