Behaviour of vis.line(update='append') in visdom 0.1.8.9

[GrumpyZhou]

I found different behavior when using vis.line under visdom 0.1.8.9 and 0.1.8.8.
When using 0.1.8.8., I can directly create a new window under a new environment by using the update='append'. This is not working samely for me when I have a newer version of visdom 0.1.8.9.

**Reproduction Steps for Visdom 0.1.8.9. and Python 3.7 **

1. Start a visdom server
2. Go to command line,

python 
>>> import visdom
>>> vis = visdom.Visdom()
Setting up a new session...
>>> vis.line(X=[1, 2], Y=[0, 2], env='main', win='test', update='append')
'test'   # It created the win with plot correctly on main env which is created by default

>>> vis.line(X=[1, 2], Y=[0, 2], env='main2', win='test', update='append')
''       # It did not create anything  (yet, it will work for 0.1.8.8.)

>>> vis.line(X=[1, 2], Y=[0, 2], env='main2', win='test')
'test'   # It created the env and win and plot,  when I don't use the update arg

>>> vis.line(X=[3, 4], Y=[0, 2], env='main2', win='test', update='append')
'test'   # It then can successfully append to plots created 

I didn't find information regarding this change, so my old code implemented under 0.1.8.8 now doesn't work properly under 0.1.8.9.
It would be nice to know whether this is the expected behavior in the latest and future version, so I can properly adapt my program, thanks.

[JackUrb]

Hi @GrumpyZhou - this change was not intentional. The final intended behavior should be create-on-append if a surface doesn't already exist for all actions, but I'm not positive why this changed recently. I'm revisiting #675 next year, after which the semantics of this should be standard.