solved: the height of graphs must be explicitly stated, in one of two ways.

1. update the style of the dcc.Graph object, ie dcc.Graph(style={"height": 600px}, figure=...}
2. change the height property in update_layout().

You could also create a CSS class to update the style. in the above case, I set the height of the left graph to 600px and the height of the left graphs to 285px, so that twice their height + margin and padding was = the height of the left graph.