I am able to change the vertical width using surface as below, but this is not quite right - because vertical width is not the same as line width when the line is not horizontal.
Finding the direction of the line and expanding the surface segment perpendicular to the line seems like it will be quite difficult.
widthrange = maxwidth-minwidth;
width = ((c ./ max(c))*widthrange) + minwidth;
h = surface(...
'XData',[x(:) x(:)],...
'YData',[y(:)-(width/2) y(:)+(width/2)],...
'ZData',zeros(length(x(:)),2),...
'CData',[c(:) c(:)],...
'FaceColor','interp',...
'EdgeColor','interp',...
'Marker','none');