inconsistent plotting behavier between x coordinate in number and dates

[barpaum]

Hi,

I've found an inconsistent behavier for plotting between number coordinate and dates coordiante.
I don't know whether it's easy to fix or it's worthless.
Please see the code below:

Part 1: initialization

from numpy import arange, sin
from matplotlib.pyplot import subplots, show
from matplotlib.dates import num2date
x=arange(0,10,0.1)
t=num2date(x+693596)
y=sin(x)

Part 2: plotting in numbers

fig,ax=subplots()
ax.plot(None,None)
ax.plot(x,y)
show()

Part 3: plotting in dates

fig,ax=subplots()
ax.plot(None,None)
ax.plot(t,y)
show()

Part 1 and Part 2 work well, but Part 3 got error of

matplotlib/dates.py in _to_ordinalf(dt)
    195             dt -= delta
    196 
--> 197     base = float(dt.toordinal())
    198     if hasattr(dt, 'hour'):
    199         base += (dt.hour / HOURS_PER_DAY + dt.minute / MINUTES_PER_DAY +

AttributeError: 'NoneType' object has no attribute 'toordinal'

[tacaswell]

Why do you want to plot None vs None? I think the issue here is that ax.plot(None, None) either does not not blow up or at least not add an artist to the canvas. If you want to add an 'empty' line I suggest using plt.polt([], [])

[barpaum]

Yes, you are right. I just do not know which one is better solution for an 'empty' line. I've just made some tests, it seems [] is a more decent way to represent an empty line in matplotlib, for example the treatment inside matplotlib legend. Thanks for your suggestion.
I've written some script to represent empty entries as None, I will update them to [].

[tacaswell]

fixed by disallowing None to be passed for x or y in #4352