Problem plotting time on X axis

[Fritz Sonnichsen]

Octave 4.0.0
I am trying to plot a time series data. I found some examples and tried 
them but they don't seem to work. I have data as shown:
9:00:38      23.9
9:00:39      23.9
.....

My code looks like this:
[dtmeT, T] = textread (filenameT, "%s %f");  %read in the data
   timeFormat='HH:MM:SS';
  ser_date=datenum(dtmeT,timeFormat); %serial date is secs since 1/1/0000
  plot(ser_date,T ,'color','k'  );
  datetick('x',timeFormat,'keepticks')
  xlen = length(dtmeT)  ;
  axis([0 xlen 20 35])

I get a time labeled axis as expected, but I do not see the plot of T.  
The data is valid-if I plot it without time formatting it works fine.

thanks
fritz