add ensureSingle & ensureSingleById lib methods
add ensureSingle & ensureSingleById lib methods

- meant to replace all our `selectAll().data([0])` calls
  by something DRYer and faster.

etpinard/plotly.js,aburato/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,plotly/plotly.js,etpinard/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,plotly/plotly.js