Data was ingested for a period of time while testing and then stopped sending to OE more than 14-days ago. Test Metric or Time Series data that does not continue to ingest into OE for 14-days is not deleted, but rather marked as "Obsolete".
After 14 days from the last ingested point, the associated time series will get marked as obsolete in DX OpenExplore. Viewing a chart for the time period when your test data was reported, then you will see no data.
Data can be included in your Dashboard or Charts if you enable the “Include Obsolete Metrics” property under the Advanced tab in your chart.
After verifying your query is correct and the data was ingested if you see no data in your chart, enable "Include Obsolete Metrics" property and data should appear as expected.