This post describes how to change the projection of your background map. (See here for explanations on basic map). Picking the right projection is a crucial step in your project, since shapes can be really different from one to the other and thus convey a different message.
See this blog post to read more about it.
Just pick the most appropriate one for your project: d3.geoAiry(), d3.geoAitoff(), d3.geoArmadillo(), d3.geoAzimuthalEqualArea(), d3.geoBaker(), d3.geoBoggs(), and many many more on the reference page.
Heavy data? It is a good practice to apply this transformation on the geoJson file directly -> avoid loading time in the browser.