test spanner with controlled gradient / dynamic volume curve indicated by grey level

[Michael Winter]

Hello...

I have tried quite a few hacks, but nothing really suitable.

What I really want is to simple draw a line beneath the staff that has a 
grey scale gradient such that for each note, I give a target value.

Most of what I have tried involves doing this in increments with 
overlapping spanners, I also tried the code in this thread:
https://lists.nongnu.org/archive/html/lilypond-user/2016-10/msg00097.html

I think the answer is doing this with svg directives since in the svg 
documentation there are directives for linear gradients, but I do not 
know how to build the function to use svg directives and such that the 
target greyscale value is reached at the x-position of a given notehead.

Also, ideally I would be able to use spacer rests to give intermediary 
values along the way, since the curve is actually not linear.

Thanks in advance and my apologies if I have been unclear.

Best,
Michael