Rethink the default axis settings for adding new metrics to timeline view.

[vingkan]

Current Behavior: When you add a new metric, the default axis alternates between left and right based on odd and even position, respectively.

Alternative Behaviors:

• Always add new metrics to the left axis.
• Always add new metrics to the same axis as the previous metric.
• Some kind of "smart" adjustment to decide where to add the metric based on its range.

Visitors can still use the selector to manually change the axis that a metric is displayed on. But we should pick a good default so that they don't accidentally create confusing visualizations, like showing two metrics of the same scale on different axes, causing them to not line up accurately; or adding two metrics of totally different magnitudes to the same axis, causing the smaller metric to not be visible.