Allows 'append' on non-existing plot for option re-use

[JackUrb]

Closes #100.

People want a blankboard so that they can use the same command in every call of an evaluation loop to plot rather than needing to instantiate the graph before they can update. Existing workarounds are doing a regular plot call on the first iteration and appends for all others, or initializing the plot with a garbage value (like 0,0 on a line) and then appending from that point. Having an option that can create or append solves the issue.

This leverages win_exists to determine whether a particular call should be a create or append when passed the 'createappend' option. It then does the correct action. Tested for both line and scatter, and existing behavior is maintained.

[lvdmaaten]

Do we really need this? Afaict it is currently perfectly well possible to write only a single call for both plot creation and updating?

# this is run this only once:
winid = None  

# ....

for _ in range(10):
    # this is run each time I'm creating or updating my plot:
    winid = vis.line(..., opt={...}, win=winid)

I'm not against adding the feature, but am generally a bit wary of bloat when things like this are being added.

[malmaud]

You can't currently pass in win=None and update='append', which is where this is useful:

From __init__.py:913:

        elif update is not None:
            assert win is not None

[lvdmaaten]

Right. So perhaps the "right" change to make here would be to allow folks to append to empty plots rather than adding a separate createappend option? This would basically mean that the createappend in the PR would replace the implementation of the current append option.

[malmaud]

Sounds fine by me.

I suppose database APIs usually make a strong distinction between updating and upserting so users are immediately alerted to logic errors in their code if they try to update a non-existing record, but in the context of visdom the increase in API complexity of the distinction might not be warranted.

[JackUrb]

Will make the change to append creating by default

[facebook-github-bot]

@JackUrb is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.