In reply to Ksharp.

Thanks for sharing. Be aware that if you use a non-sparse data set, the white cells can overplot the black cells. This can result in a misleading image. To prevent that, you need to make sure that the graph has as many pixels in the 'Observation' dimension. Alternatively, you can use a WHERE clause or IF statement such as

where obs=1 OR col1=1;

to convert from dense to sparse format.