Support for unconventional y-axis units such as `uA cm-2 M-1`

https://github.com/echemdb/electrochemistry-data/pull/43 has units on the y-axis denoted as `uA cm-2 M-1`. These are not supported by unitpackage, nor is `M` supported by astropy. Currently the unit is given as `uA / cm2` in the SVG, which should be changed once this unit is supported.