Matplotlib-3.4.0 compatibility (#4878)
Matplotlib-3.4.0 compatibility (#4878)

* Matplotlib-3.4.0 compatibility 

following this API change https://github.com/matplotlib/matplotlib/pull/18544

* Update util.py

* Update util.py
ioam/holoviews,ioam/holoviews,ioam/holoviews