def plot_null_matrix(df, figsize=(18,15)):    
    # initiate the figure    
    plt.figure(figsize=figsize)    
    # create a boolean dataframe based on whether values are null    
    df_null = df.isnull()    
    # create a heatmap of the boolean dataframe    
    sns.heatmap(~df_null, cbar=False, yticklabels=False)    
    plt.show()