Plotting incorrectly reversed when using dates as x-axis
Sometimes I have to read in spreadsheets containing data with the most
recent observations first. If I try to plot these without re-sorting
first, the graph appears _backwards_. The dates on the x-axis are in
ascending order, but the data are plotted from left to right in the same
order they appeared in the spreadsheet. Since there's no warning that the
labels on the axis don't correspond to the data being plotted, this can
cause some embarrassment! This only happens when I use timestamps; dates
as integers work fine.

For example, this works fine (dates reversed, stored as int)

df2 = DataFrame({'Date' : [1998, 1997, 1996], 'Data' : [9, 8,
7]}).set_index('Date')
df2.plot()

This doesn't (same data set, but dates stored as timestamp instead):

from dateutil import parser
df = DataFrame({'Date' : ['1998-01-01', '1997-01-01', '1996-01-01'],
'Data' : [9, 8, 7]})
df.index = df.Date.map(parser.parse)
del df['Date']
df.plot()

I can get around the problem by sorting the data set before plotting, but
I don't always remember, and it definitely seems odd that the graph ends
up displaying something that is not actually in the original data set...
