Whether it's a Series, NumPy array, or list doesn't matter.Just use df.groupby(), passing the DatetimeIndex and an optional drill down column. See many more examples on plotting data directly from dataframes here: Pandas Dataframe: Plot Examples with Matplotlib and Pyplot Plot the number of visits a website had, per day and using another column (in this case browser) as drill down.
The axis labels are collectively called index.Pandas Series is nothing but a column in an excel sheet. You should use the simplest data structure that meets your needs. An list, numpy array, dict can be turned into a pandas series. You can create a series by calling pandas.Series (). It can hold data of many types including objects, floats, strings and integers.