I am trying to plot a topomap of data that I calculated for each channel in some MEG data. So for example for channel: MEG0113 I may have a value of 0.5.
Stepping back for a minute, why does evoked.plot_topomap itself automatically not just do the right thing for you? Personally I would find it much easier and robust to make an EvokedArray of your data with a suitable info and let things be taken care of automatically. I would not use layout at all for this stuff.
Also, the topomap interpretation for un-combined grad channels for VectorView is weird/difficult because each close channel pair will actually measure orthogonal directions. The native evoked.plot_topomap will do a grad pair norm, which helps you in that regard. (plot_topo, on the other hand, could be used if you really want to keep the two directions separate, since it plots tiny time courses rather than doing a color interpolation.)