
You need to have at least two matrix objects in the same matrix sheet. If you want all the matrix objects included in the same layer of the graph, you should select the first matrix object.
Activate the matrix sheet and select a matrix object(do not select the last one).
From the menu, choose .
or
Click the Multiple Color Map Surfaces button on the 3D and Contour Graphs toolbar.

CMAP.OTP (installed to the Origin program folder).