code: function buildContainerGroups() {
              let container = svg
                .append('g')
                  .classed('legend-container-group', true)
                  .attr('transform', `translate(${margin.left},${margin.top})`);
  
              container
                .append('g')
                  .classed('legend-group', true);
          }
docs: Builds containers for the legend
  Also applies the Margin convention
  @private