Write the data out as base64 binary
        
        Args:
          output (file-like object): Output to write figure to.

def write_data(self, output):
        """Write the data out as base64 binary
        
        Args:
          output (file-like object): Output to write figure to.
        """
        if self.figure:
            canvas = FigureCanvas(self.figure)
            self.figure.savefig(output, format='png', bbox_inches='tight')
            output.seek(0)
            return output.getvalue()
        return None