Allow multiple Explore visualizations to share one chart and Y axis

When a query has multiple visualizations, each is currently displayed in a separate chart with its own Y axis. It's difficult to compare the visualizations this way.

Add an option to combine the visualizations into a single chart with a shared Y axis, while keeping each series identifiable in the legend.

Acceptance criteria:
• Users can switch between separate charts and a combined chart.
• Combined visualizations share one Y axis when their units are compatible.
• Series remain distinguishable in the legend.
• Visualizations with incompatible units are handled clearly, such as by using separate axes or preventing combination.

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via neil.
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