Split vx_n_vs_mu plots in InDetPhysValMonitoring
Fixes the vx_n_vs_mu
DCube plot for IDPVM Run4 tests, broken by !68243 (merged). Follow up to !68400 (merged).
Fixes the vx_n_vs_mu
DCube plot for IDPVM Run4 tests, broken by !68243 (merged). Follow up to !68400 (merged).