function coerce2(attr, dflt) {
        return Lib.coerce2(containerIn, containerOut, opts.attributes, attr, dflt);
    }
@param {object} opts :
  - dfltColor {string} : default axis color
  - bgColor {string} : combined subplot bg color
  - blend {number, optional} : blend percentage (to compute dflt grid color)
  - showLine {boolean} : show line by default
  - showGrid {boolean} : show grid by default
  - noZeroLine {boolean} : don't coerce zeroline* attributes
  - attributes {object} : attribute object associated with input containers