Cumulative charts for Disk Details were not showing any data if metrics were returning 'NaN' results.

STR:

• Install PMM2 server
• Add for monitoring an instance with two disk devices
• Keep one disk in an idle state for a period of time
• Open Disk Details dashboard
• Set start of dashboard time range equal to start of the disk idle period
  Given result:

  One device selected	All devices selected

As you may see the chart is empty if "All" devices are selected.
The reason of such thing is that the second device had no activity and used formula return null value.

Expected result:
Metrics with NaN values shouldn't affect cumulative charts.