Hello, hello 
So the problem is that there are no values (result) for those time period covered by your query. If you would like to remove the gap in your graph on Grafana side, you can choose Connected null values Panel edit -> Panel -> Display -> Stacking and null value. “Connected” means that if you get a null value, join the preceding and succeeding values with a straight line, effectively pretending that you didn’t expect to get a measurement where the null appeared.
Here is also couple of resources on how to troubleshoot/what to do on Prometheus side:
Prometheus community resources
Regarding:
Strangely enough, the graph shows a line, but the inspector doesn’t show numbers…
Based on your screenshot, the inspector doesn’t show the numbers cause at that time (12:40 - 12:48), there are no data and they are not shown in graph either.