There can be several reasons for it:
1. Go to configuration page and check whether your code and configuration are there or not. If it is saved then check whether the variable name is same in code and table on the right side of the code editor.
2. If the above step is correct then go home and in the devices tab, there would be a cloud icon named deploy configuration. Click on that icon if the success message comes "Configuration deployed successfully" then click on view device icon and see whether data is coming or not on the graph.
3. Check whether the blank graph is shown on the page or not and also the data count.
4. If still data is not coming, right click then select inspect element and then click on the console and see what the error is.