def plot(self,columns,time_column='index'):
        """
        calculates the slope of the selected columns

        Parameters
        ----------
        columns : array of strings
            columns to plot
        time_column : str
            time used for calculation; default to 'h'
        """
        fig = plt.figure(figsize=(10,6))
        ax = fig.add_subplot(111)
        if time_column=='index':
            for column in columns:
                ax.plot(self.time,self.data[column],marker='o')
        else:
            for column in columns:
                ax.plot(self.data[time_column],self.data[column],marker='o')
        ax.legend()

        return fig,ax