Bar chart is invisible when change the time period


We use grafana 6.4.3 and postgres .
We use the graph panel to show some alarm data in bar chart. For this panel, the data is not coming evenly, and sometimes we see the data is invisible, but with lines and points, it shows fine. And when we narrow down the time period, sometimes it shows. Is there specific requirement to use the bar chart?

Below is the some screenshots.