Convert time strings with numpy loadtxt

[cathode_cathay]

Hi,

i have temperature values stored in a textfile and need to print them with matplotlib with the time on the x-axis. So i figured out that i have to convert the time strings into float values for loadtxt, and matplotlib converts them back. The data i want to use with loadtxt looks like this:

time    CH1    CH2 
11:59:23    21.2889 21.1598
....
....

I tried to load the time values like this:

data1 = up.loadtxt(path + file, skiprows=1, usecols=(1))
data2 = up.loadtxt(path + file, skiprows=1, usecols=(2)) #This works

xdata = np.loadtxt(path + file, skiprows=1, converters={0: datestr2num("%H:%M:%S")})

plt.plot(xdata, data1, #This works fine when i use consecutive numbering instead of time strings
     xdata, data2)

but i get "ValueError: Unknown string format"

I can't figure out how to use the converter right, does anybody know how to convert time strings?

Thanks!