Don't show y=0 unless there is a data point near y=0

[openjck]

Su brought this up in Slack.

Some charts look essentially flat when they aren't because y=0 is shown whether or not there is a data point at y=0. We should just be aware that changing this will exaggerate any trends, including downward trends, whether temporary or otherwise.

[openjck]

Unfortunately, I can't seem to find a good way to achieve this right now. There's a bug in metrics-graphics that creates lots of empty space above the line when y=0 is not shown, which makes the line appear relatively flat and thus defeats the purpose of omitting y=0. Also, using max_y as a workaround causes parts of the line to become invisible.

[openjck]

It may be possible to work around this.