Immutable mode

Along with Plotly.react, it would be nice if we would never mutate gd.data and gd.layout, just make new changed objects and alert the user to the change. We can't make this the default behavior in v1 since some users depend on gd.data and gd.layout keeping the identity of the objects they initially passed in - though there are some cases already where they are not. But that rule holds well enough that some folks depend on it, so immutability would have to be a config argument.

In general all of the plot modification methods except Plotly.react (Plotly.restyle, relayout, update, extendTraces etc..) modify gd.data and gd.layout in place to make the requested updates. Most (but not all) then emit an event you can use to see that the plot state has changed. But also there are modifications that happen automatically during drawing the plot, principally (if not entirely) pushing auto values (axis ranges, colorscale ranges) back to gd.data and gd.layout so that future minor updates don't need to recalculate these. This too is something we might be able to avoid doing, but probably not until v2.

So there are two things we should do:

• Make all updates to gd.data and gd.layout immutably, conditional on a config argument.
• Streamline the way we alert the user that the state has changed, so that they can feed this new state back into their app state. It seems like once we have immutable changes, it might be possible without any changes to the events from plotly.js... the react wrapper would have to bind to all the events that might carry state changes initiated from the UI, and also check after every update initiated by the wrapper itself, whether gd.data === prevData and gd.layout === prevLayout... but seems like it would be much easier for the react wrapper if there were a single "new state" event that collects all of these.

See https://github.com/plotly/react-plotly.js/issues/43

Start by tracing Plotly.react, Plotly.restyle, relayout, update, and extendTraces, then review the linked react-plotly.js issue #43. Determine how updates and drawing currently mutate gd.data and gd.layout, including automatic values and state-change events. Done means immutable updates are conditional on a config argument and state changes have a streamlined notification path.