Your local check has to explicitly return performance data (metrics). For example:
0 mylocalcheck myvalue=12 My local check has found a value of 12 and all is well.
Only the “myvalue=12” is taken as metrics. The “…value of 12 …” is just free human-readable text.
See the manual for details on the output format: https://checkmk.com/cms_localchecks.html
The metrics returned by this check (“myvalue”, currently at 12) are automatically graphed using the default template, no further configuration is required for this.