docstr: Format value for dual value display.
docstr_tok: ["Format","value","for","dual","value","display","."]
code: def _value_format(self, value):
      """
      Format value for dual value display.
      """
      return "%s: %s" % (
          self._x_format(value[0]),
          self._y_format(value[1]),
      )
  
imports: from pygal.graph.graph import Graph
  from pygal.util import compute_scale, cut
label_desc: Use image node_id 1 for calling the Dual obj's underlying member method code with example usage: obj._value_format(value) and returns: unknown