Unable to create custom chart with biometric "blood pressure"

I would like to create a custom chart with the biometric blood pressure plotted against sodium. When creating a custom chart, the only options available are "systolic blood pressure" and "diastolic blood pressure" which do not pull in automatically reported BP measurements from external devices (these are reported as combination BP, e.g. 120/80). The default BP vs HR chart works just fine, but there is no way to create a custom chart using this BP metric.