Since you replaced lodash _.merge by a custom Object.assign call, you have a pretty bad regression on default styles because lodash ._merge does more than Object.assign (it works recursively on nested objects).
Default styles are not correctly merge and many of my graphs look different after update. I will do my own merge for the moment but you should fix that 😄
Oh yeah did not thought about nested object.
Will fix it.
+1 I'm also working around it at the moment by using deepmerge (https://github.com/KyleAMathews/deepmerge)
Will with fixed with #191
Replaced object.assign with deepmerge as it seems to be a nice small and lightweight lib :3
Fixed with version 2.8.4