Syntax:
spyx(x,y,S)

This programs uses the SPY function, which is used to plot marks on the non zero matrix elements (even on NaN's), but uses the user-specified axes (x,y) instead of rows and columns indexes (check the screenshot where

S = [
5.6 9.9 1.8 9.1 8.9 0 0 7.5 6.9 0
8.5 5.4 3.6 1.0 0.7 0 0 0 0 0
3.5 7.1 0 7.5 0 0 0.8 0 4.0 8.9
4.5 NaN NaN 7.4 0.5 3.3 7.8 0 0.6 0
0.5 NaN NaN 0 0 0 0 0 7.8 0
1.8 0 1.8 0 0 0.6 0 5.3 3.4 0
6.6 0 2.1 6.0 0 3.0 1.1 7.3 6.1 0
3.3 7.6 9.1 0 NaN 0 NaN 0 7.4 0.9
0 8.2 6.8 1.3 0 5.1 0 7.8 0 0
1.2 1.0 4.7 0 0 0 0 2.9 0 NaN
];