Update copies consistent subset of {x,y,z}data to children of the stairs hggoup. (Bug # 39234) * scripts/plot/stairs.m: When a listener for {x,y,z}data triggers an update, copy {x,y,z}data(1:M,1:N) to the hggroup's children, where M/N are the minimum number of rows/columns among the matrices, {x,y,z}data. Add demo.

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