I would prefer simply giving cell with weight another background color. Preferably a darker variant of the existing cell color. You would then be able see if the cell has a weight or not. That would meet your requirement.

There are two reasons I prefer not to use the idea of a colored bar:

1. It is not clear what weight would represent a maximum filled bar.
2. Small weight may lead to a to small bar. Again invisible just as the numbers.
3. I used color bar in the DSM once, but found it too much visual clutter and therefore chose another approach in the end.

I you agree I can make an update next weekend.

Here's an example (at the minimum zoom level) where the bar represents quantiles of 10%. Even dependency weight 1 is visible and the bar is full for the top 10%. This scales nicely with what the matrix currently displays. The exact number is still visible in the cell and of course in the tooltip. Granted, the black bar is a bit intense, but dark grey would also work.

And here's a version where the cells themselves are darkened (binary, independent of actual weight). The disadvantages I see here are

1. Many different background colours. The basic 4, and darkened for weights, lightened for the selection and then there's the hovered cell. What happens when a banding algorithm is added?
2. The contrast between background and text gets too low for dark cells, so you'd have to change the letter colouring which makes the whole display less eye-friendly.
3. Neighbouring cells with different backgrounds but both darkened get very similar. The user can only distinguish them because the mind draws boxes around other cells without weight, whose colour is more distinguisable.
   

Here's the bar again with dark grey instead of black. Looks better, IMHO.

Thanks for all effort put in the 4 pull requests. Currently not having much time available. Will look into these when I have some time.

I you have other feedback on the usage of the tool please let me know. I see you added TODO in the code.