graph: clear the graph when there is nothing to draw

[oliver-sanders]

If there is nothing to draw, the graph view currently preserves the last valid graph. This is a little inconsistent, other views go blank.

I think there have been some issue with differentiating between empty and error states once?

We should allow the graph to go blank and, ideally, leave a placeholder where the graph would be to explain this, however, that is a different issue - #332.

[MetRonnie]

I think this is a bug Something isn't working because of this from cylc/cylc-uiserver#630

An example. Note the workflow is stopped after having completed, but the task still claims to be running.

[oliver-sanders]

I think this is a bug because of this from cylc/cylc-uiserver#630

I don't think that's right.

• This issue is about clearing the graph when the task pool is empty.
• The issue you linked was about the task state not being updated before the workflow shut down.

Note that other views (e.g. the tree view) will retain the workflows last known state after it has shut down (similar to the graph view) so if the task has been left in the wrong state, that is a different bug. Possible causes:

• Scheduler did not send the task state transition before shutting down.
• UIS did not forward the task state transition due to the workflow shutting down.
• UI data store did not enact the task state transition due to the presence of the shutdown delta.
• UI Graph view became decoupled from the UI data store (so the task state change did not happen).