ID              Date       Element  Data_Value
0   USW00094889     2014-11-12  TMAX    22
1   USC00208972     2009-04-29  TMIN    56
2   USC00200032     2008-05-26  TMAX    278
3   USC00205563     2005-11-11  TMAX    139
4   USC00200230     2014-02-27  TMAX    -106

res = df.groupby('Date')['Data_Value'].agg(['max','min']).reset_index()

x=res[['Date']]
y1=res[['max']]
y2=res[['min']]
fig = plt.figure()
plt.plot_date(x, y1, fmt='-r')
plt.plot_date(x, y2, fmt='-g')