Drawing a regression line with a timeseries My code is using seaborn to draw a regression line with a timeseries data as the x axis, and whatever you want the y-axis to be. Since seaborn does not support this function directly, a dummy column has to be made first based on the timeseries (i.e. instead of using 2016-05-01, use 1,2,3,4,5... to represent the date. Then, plot the regression line using 1,2,3... as the x-axis and replace the 1,2,3 label with 2016-05-01, 2016-05-02...

def regplot_timeseries(df:pd.DataFrame, time_col:str, data_col:str, figsize=(14, 6), xlabel = '', ylabel = ''):
    # make a copy of the incoming data (think of the incoming data as an excel sheet visually)
    dfc = df.copy()

    # if it is a index, treat it slightly differently 
    # add a column to the copy of the excel sheet for plotting purpose 
    if time_col == 'index':
        dfc = dfc.sort_index()
        dfc['date_f'] = pd.factorize(dfc.index)[0] + 1
        mapping = dict(zip(dfc['date_f'], dfc.index.date))
    else:
        dfc = dfc.sort_values(time_col_str)
        dfc['date_f'] = pd.factorize(dfc[time_col])[0] + 1
        mapping = dict(zip(dfc['date_f'], dfc[time_col].dt.date))

    # plotting
    fig, ax = plt.subplots()
    fig.set_size_inches(figsize[0], figsize[1])
    sns.regplot('date_f', data_col, data=dfc, ax=ax)
    labels = pd.Series(ax.get_xticks()).map(mapping).fillna('')
    ax.set_xticklabels(labels)
    ax.set_xlabel(xlabel)
    ax.set_ylabel(ylabel)

    # delete the copy to free up memory
    del dfc


I am new to functional programming, and from what I read, I shouldn't alter the data being passed in. In this case, I shouldn't alter df, so I made a copy instead and deleted the copy at the end of the function. Is this the right to do it?
You don't need dfc, dfc.sort_index() and dfc.sort_values(time_col_str) both make a copy of the data. They don't perform an in-place sort.
del dfc isn't needed, Python will perform that action at the end of the function anyway. Also I don't think it's doing what you think it's doing. I may be wrong, but since variables in Python are like names, when you're using del, you're just removing one name tag to the object.


  Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a global statement in the same code block.

In my personal opinion, Python doesn't lend itself well to functional programming. Yes you can do FP in it, but it tends to start looking very ugly very fast. Don't care about whether something is FP or not, just make it work.
I don't really know what your code is doing, but the if-else and the 'plotting' should probably be in two separate functions.