documentclass{standalone}
  usepackage{pgfplots}
  usepgfplotslibrary{dateplot}
  pgfplotsset{compat=1.9}

begin{document}
 begin{tikzpicture}
  begin{axis}[date coordinates in=x,date ZERO=2013-08-18,
               xticklabel=month-day,ymin=0,ymax=10000]
   addplot coordinates {
    (2013-08-18, 14)
    (2013-08-25, 245)
    (2013-08-31, 3412)
    (2013-09-04, 4567)
    (2013-09-05, 5001)
    (2013-09-12, 6891)
    (2013-09-13, 7456)
    (2013-09-15, 8234)
    (2013-09-21, 9456)
   };
   addplot [red] expression {1/(1+exp(-x))};  % <-- fails if uncommented
  end{axis}
 end{tikzpicture}
end{document}

documentclass{standalone}

usepackage{pgfplots}
usepgfplotslibrary{dateplot}
pgfplotsset{compat=1.8}

begin{document}
 begin{tikzpicture}
  begin{axis}[date coordinates in=x,date ZERO=2013-08-18,
               xticklabel=month-day,ymin=0,ymax=10000,
       xmin=2013-08-18,
       xmax=2013-09-21]
   addplot coordinates {
    (2013-08-18, 14)
    (2013-08-25, 245)
    (2013-08-31, 3412)
    (2013-09-04, 4567)
    (2013-09-05, 5001)
    (2013-09-12, 6891)
    (2013-09-13, 7456)
    (2013-09-15, 8234)
    (2013-09-21, 9456)
   };
   %addplot [red] expression {1/(1+exp(-x))};  % <-- fails if uncommented
  end{axis}
 end{tikzpicture}
end{document}

date,days.since,function.value
2013-08-18,0.00,13
2013-08-19,1.00,15.87823586
2013-08-20,2.00,19.39372107
2013-08-21,3.00,23.68754441
2013-08-22,4.00,28.93203207
2013-08-23,5.00,35.33766377
2013-08-24,6.00,43.16152
2013-08-25,7.00,52.71759957
2013-08-26,8.00,64.38942152
2013-08-27,9.00,78.64541704
2013-08-28,10.00,96.05772929
2013-08-29,11.00,117.3251755
2013-08-30,12.00,143.3012929
2013-08-31,13.00,175.0285945
2013-09-01,14.00,213.780408
2013-09-02,15.00,261.11198
2013-09-03,16.00,318.9228926
2013-09-04,17.00,389.5333006
2013-09-05,18.00,475.7770478
2013-09-06,19.00,581.1153984
2013-09-07,20.00,709.7759504
2013-09-08,21.00,866.9223035
2013-09-09,22.00,1058.861293
2013-09-10,23.00,1293.296103
2013-09-11,24.00,1579.635428
2013-09-12,25.00,1929.371068
2013-09-13,26.00,2356.539144
2013-09-14,27.00,2878.283411
2013-09-15,28.00,3515.543297
2013-09-16,29.00,4293.894279
2013-09-17,30.00,5244.574315
2013-09-18,31.00,6405.737534
2013-09-19,32.00,7823.985492
2013-09-20,33.00,9556.23746
2013-09-21,34.00,11672.01479

documentclass{standalone}

usepackage{pgfplots}
usepgfplotslibrary{dateplot}
pgfplotsset{compat=1.8}

begin{document}
 begin{tikzpicture}
  begin{axis}[date coordinates in=x,date ZERO=2013-08-18,
               xticklabel=month-day,ymin=0,ymax=10000,
       xmin=2013-08-18,
       xmax=2013-09-21]
   addplot coordinates {
    (2013-08-18, 14)
    (2013-08-25, 245)
    (2013-08-31, 3412)
    (2013-09-04, 4567)
    (2013-09-05, 5001)
    (2013-09-12, 6891)
    (2013-09-13, 7456)
    (2013-09-15, 8234)
    (2013-09-21, 9456)
   };
   addplot table [x=date,y=function.value,col sep=comma]
       {2014-01-19.csv};
  end{axis}
 end{tikzpicture}
end{document}

documentclass[12pt,a4paper]{article}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}

usepackage{graphicx}
usepackage{pgfplots}

usepgfplotslibrary{dateplot}
pgfplotsset{compat=1.8}

begin{document}

  begin{tikzpicture}
  begin{axis}[
    title={La dinámica de los precios del petróleo, USD/barril},
    date coordinates in=x,
    xticklabel style={rotate=90},
    enlarge x limits=false,
    xticklabel={day.month.year},
    date ZERO=2015-10-01,
    xmin=2015-10-01,
    xmax=2015-12-31,
    xtick={2015-10-01,2015-10-15,2015-11-01,2015-11-15,2015-12-01,2015-12-15,2015-12-31}]

  addplot[blue] table[x=date,y=brent]{oil_prices.txt};
  addplot[red] table[x=date,y=wti]{oil_prices.txt};
  legend{Petróleo Brent,Petróleo WTI}

  end{axis}
  end{tikzpicture}
end{document}