new_contents: import pandas as pd
  from matplotlib import pyplos as plt
  
  
  def plot_df(df,col,cat=None,linestyle=None,marker=None):
      '''df must be a pandas DataFrame, 
      col and cat should be in df.columns and
      cat is a categorical variable
      TO-DO: pass *args and **kwargs to the plot function
      '''
      if col not in df.columns:
          return
      if cat is None:
          df[col].plot()
      else:
          options=df[cat].unique()
          for opt in options:
              df[col][df[cat]==opt].plot(label=opt,linestyle=linestyle,marker=marker)
              plt.legend()
subject: add utilities.py file, some useful functions