plot3d discrete problem

This plot used to work in maxima 5.18.1 but does not now work.

(%i1) M: matrix([2, 2, 3, 5], [2, 1, 3, 2], [2, 2, 3, 4],

?????? [2, 2, 3, 3], [1,1,1,4])$
(%i2) M;
(%o2) matrix([2,2,3,5],[2,1,3,2],[2,2,3,4],[2,2,3,3],[1,1,1,4])
(%i3) f(x, y) := float (M [?round(x), ?round(y)])$
(%i4) plot3d (f, [x, 1, 5], [y, 1, 4], ['grid, 4, 4], ['xlabel, "x-axis"], 
[plot_format, openmath])$


Thanks for the workaround for my previous post, is there one for this as well?

Regards, Rob.