BUG: mixed freq timeseries plotting with shared axes (GH13341)
Closes #13341, partly closes #14322 (that example still does not work when first plotting the irregular series)

cc @sinhrks @TomAugspurger 

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14330?src=pr) is 85.26% (diff: 93.75%)


> No coverage report found for **master** at b1d9599.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [b1d9599...b3537e2](https://codecov.io/gh/pandas-dev/pandas/compare/b1d95990b4bad8e6dd555607e98dad690cfd59d6...b3537e2f65db7698089109caad87f393225f5bb0?src=pr) @jorisvandenbossche @TomAugspurger ? Rebased, moved to 0.19.2 and addressed Tom's comment. 
Should be good to merge I think when green.