Choropleth equirectangular projection and straight lines


I’m trying to draw rectangular polygons on a choropleth map, using the equirectangular projection, with polygon edges corresponding to longitude/latitude… unfortunately the edges always seem to be drawn as shortest-distance paths, and I’m not sure if there is any around this, other than adding lots of points to the polygons (which gets ugly of course when zooming in). the following shows my problem:

is there some well hidden option perhaps to disable using shortest-distance paths?

thanks, cheers,