meshm - R parameter

I have a bathymetry netCDF variable that is not square (3027x8796, where the depth values correspond to (latitude x longitude).

The meshm documentation explains what different forms the 'R' parameter can take, and I think I need the 'referencing matrix' version, where:

If R is a referencing matrix, it must be 3-by-2 and transform raster row and column indices to/from geographic coordinates according to:

[lon lat] = [row col 1] * R

I sort of follow the principle, but I have no idea how to construct this (perhaps it's been a long day), and the documentation doesn't give an example of what it means.

I think this is what is causing the white space I'm seeing in my 'map':

axesm('mercator','MapLonLimit',[-17.7063 18.9396], 'MapLatLimit',[65.7229 78.3313]);
% set(gca, 'xlim', [-17.7063 18.9396]); set(gca, 'ylim', [65.7229 78.3313]);
[lattrkgc,lontrkgc] = track2(lat1,lon1,lat2,lon2);
[lattrkrh,lontrkrh] = track2('rh',lat1,lon1,lat2,lon2);
R = [3027/13 78.3313 -17.7063];
meshm(bathy, R);
demcmap(bathy);
plotm(lattrkgc, lontrkgc, 'r');
plotm(lattrkrh, lontrkrh, 'g');

Producing:

What's the best way of getting rid of this white space (and also getting axes labels, as the set() command doesn't seem to be achieving anything other than squashing the map despite having matching limits). Thanks.