TODO: Add a date range to the plotting function

[rjalexa]

def regplot_of_df(df, from_date, to_date) could be an idea with those two parameters; if omitted it would plot ALL df values, if instantiated (also independently) it would select the corresponding rows

[rjalexa]

from=date (check if date is good, if omitted use 01/01/1970)
to=date (check date is good, if omitted use today))
check from is < than to
last=DD (DD days going back from today