I tried to do the job through the old way and it's working.
But I have the latest version (0.91.2). Is it possible that my version wasn't successfully installed and that an older one is doing the job ?
Anyway, thanks a lot for the (fast) help.

2008/4/29 John Hunter <jdh2358@gmail.com>:
On Tue, Apr 29, 2008 at 12:10 PM, Johan Mazel <johan.mazel@gmail.com> wrote:
> When you wrote "import matplotlib.ticker", you meant "from matplotlib.dates
> import ticker" ?

No sorry, I meant "import matplotlib.ticker as ticker"

> I think I have a bug or something because I get an error like "ValueError:
> setting an array element with a sequence." with a lot of debug stuff from
> python and MatPlotLib
>  The line concerned is the one where I do plot(datetime_list,data_list).
> It's very strange because just before I use "my_datetime.strftime("%d %H %M
> %S")" with my_datetime as an element of my list and it works perfectly.

Possibly your matplotlib version is a bit old.  With the latest
release (0.91.2) you can pass sequences of native datetimes in.  For
older versions, you had to convert your dates to numbers first:

import matplotlib.dates as mpldates

d = mpldates.date2num(datetime_list)
ax.plot_date(d, ydata)