maint: Clean up m-files to follow Octave coding conventions. Try to trim long lines to < 80 chars. Use '##' for single line comments. Use '(...)' around tests for if/elseif/switch/while. Abut cell indexing operator '{' next to variable. Abut array indexing operator '(' next to variable. Use space between negation operator '!' and following expression. Use two newlines between endfunction and start of %!test or %!demo code. Remove unnecessary parens grouping between short-circuit operators. Remove stray extra spaces (typos) between variables and assignment operators. Remove stray extra spaces from ends of lines.

--- a/scripts/plot/util/ndgrid.m
+++ b/scripts/plot/util/ndgrid.m
@@ -121,7 +121,7 @@
 %!assert (ndgrid ([]), zeros(0,1))
 %!assert (ndgrid ([], []), zeros(0,0))
 
-%% Test input validation
+## Test input validation
 %!error ndgrid ()
 %!error <wrong number of input arguments> [a,b,c] = ndgrid (1:3,1:3)
 %!error <arguments must be vectors> ndgrid (ones (2,2))