Graphs plugin selects wrong tag when switching runs

The graph plugin demo added in #4469 has the following data in the
graphs plugin’s dropdown menus:

run keras/train
	tag Default
	tag batch/2
	tag keras
run profile
	tag prof_f
	tag prof_g
run tagged
	tag Default
	tag step_0000
	tag step_0001
	tag step_0002

When the plugin first loads, it selects run keras/train with the
default tag. If you switch to run profile, the tag selector stays at
Default. Expanding the “Tag” dropdown shows the two options prof_f
and prof_g, as desired, but selecting prof_f does nothing. If you
select prof_g, then you can select prof_f. Select prof_g again,
and switch the run to tagged. This resets the tag to prof_f. And it
stays at prof_f if you then set the run back to keras/train.

Playing around a bit more, it looks like maybe the pattern is “when you
switch the run, it sets the tag to the first tag of the most recent run
for which you manually selected a tag”?

Can repro in 2.4.0, 2.3.0. Tried to install older TensorFlows, but I’m
out of disk space. These TensorFlows aren’t cheap, you know.

Reproduce the issue in the graphs plugin demo using the listed runs and tags, then trace the run and tag dropdown state as the selected run changes. Done means selecting any displayed tag works immediately and switching runs selects an appropriate available tag without retaining a tag from another run.