its pretty basic, but at the same time a bit vage whats happening.
i have been checking out this d3.select stuff now for a while, and i still mess it up.
here a codepen link where i try to implement stuff https://codepen.io/ronstarcool/pen/OoMWQV?editors=1010
it has 3 buttons.
expected: click button, see red squars or blue, or yellow.
current reality: click button, see black square. click again, turns red
how to fix this?