What problem does this feature solve?

When updating certain aspects of a graph type layout (like opacity) the whole chart gets redrawn, which causes a lot of jumping around in the graph. If it was possible to force a seed to how the graph layout is calculated, or at least export the previous coordinates, so they can be re-supplied, keeping existing layout

What does the proposed API look like?

I imagine this could be provided via an export from the Echarts instance class.