So XY Plots in PI Vision require two specific things to be in place - properly configured Kerberos delegation and a CA or Windows domain issued SSL certificate. If Kerberos isn't properly configured or you are only using the default self-signed SSL certificate then this could be the reason your symbol doesn't display data. These are the first two things I would want to verify before digging any further at this point.
As John Messinger mentioned, XY-Plots require third-party certificates; they cannot use self-signed certificates created during the Installation. Else you will see a error message at top right corner of PI Vision display: "PI Web API Error : The PI Web API server could not be reached. Please contact your system administrator or refer to the PI Web API troubleshooting section in the PI Web API User Guide for possible causes and solutions"
You can get a third-party digital certificate but you can also create a domain-trusted certificate by coordinating with your IT department. We have an informative PI Square post for your reference: How to Create a Certificate Using Your Enterprise CA for PI Web-based Products