[nvd3] fix single metric showing up in legend (#3563)
[nvd3] fix single metric showing up in legend (#3563)


airbnb/superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,airbnb/superset,alanmcruickshank/superset-dev,mistercrunch/panoramix,zhouyao1994/incubator-superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,mistercrunch/panoramix,timifasubaa/incubator-superset,dmigo/incubator-superset,timifasubaa/incubator-superset,apache/incubator-superset,apache/incubator-superset,airbnb/superset,airbnb/superset,airbnb/caravel,dmigo/incubator-superset,alanmcruickshank/superset-dev,alanmcruickshank/superset-dev,apache/incubator-superset,dmigo/incubator-superset,airbnb/caravel,alanmcruickshank/superset-dev,airbnb/caravel,apache/incubator-superset,timifasubaa/incubator-superset,dmigo/incubator-superset,zhouyao1994/incubator-superset,timifasubaa/incubator-superset