plotting time series in Octave

[financial engineer]

I have the following data and I am trying to plot the time series and an overlay of signals on it

2011-12-06 02:15:00," 3.113830e-01"," 1.909586e+02","U","8575.000","   0.000","   0.000","   0","0.887775615"
2011-12-06 02:16:00," 2.630128e-01"," 1.848320e+02","U","8575.000","   0.000","   0.000","   0","0.891741773"
2011-12-06 02:17:00," 2.194390e-01"," 1.784746e+02","B","8575.000","8554.142","8556.332","8550","0.892192117"
2011-12-06 02:18:00," 1.750296e-01"," 1.727000e+02","B","8575.000","8554.287","8556.518","8550","0.894445120"
2011-12-06 02:19:00,"-2.392036e+00"," 1.558317e+02","U","8580.000","   0.000","   0.000","   0","0.898210621"
2011-12-06 02:20:00,"-3.538674e+00"," 3.513018e+02","U","8580.000","   0.000","   0.000","   0","0.898689659"
2011-12-06 02:21:00,"-5.469638e+00"," 6.806506e+02","U","8582.500","   0.000","   0.000","   0","0.901698147"

I have converted the data into a matrix:
C=csv2cell("test.csv",",")

and then ran the following command:
octave-3.2.4:23> C{3:4,1:9}
ans = 2011-12-06 02:17:00
ans = 2011-12-06 02:18:00
ans =  0.21944
ans =  0.17503
ans =  178.47
ans =  172.70
ans = B
ans = B
ans =  8575
ans =  8575
ans =  8554.1
ans =  8554.3
ans =  8556.3
ans =  8556.5
ans =  8550
ans =  8550
ans =  0.89219
ans =  0.89445

I tried to plot using the following command:
octave-3.2.4:22> plot(C{3:4,5},C{3:4,6})

and I get a blank graph....obviously I am doing something silly....

my ultimate goal is:
1. plot a time series with the first column as my x-axis and the 5th column as my y-axis
2. overlay points on the time series graph (at the price) to indicate a B whenever a B shows up in the 4th column

Can someone please help me make the first correct step so I can move to the second step. Thanks!