BUG: timeseries subplots may display unnecessary minor ticklabels

[sinhrks]

Related to #7457. The fix was incomplete because it only hides major ticklabels, not minor ticklabels. This causes incorrect result in time-series plot.

(And another problem is rot default is not applied to minor ticks. I'll check this separatelly)

df = pd.DataFrame(np.random.randn(10, 4), index=pd.date_range(start='2014-07-01', freq='M', periods=10))
df.plot(subplots=True)

Current result:

Minor ticklabels of top 3 axes are not hidden.

Result after fix:

[sinhrks]

Ah, this is not good as vertical grids are hidden. Leave it a while...

[jreback]

@sinhrks this is a bug-fix or enhancement?

[sinhrks]

Fixed to display grid lines, and only hides major and minor ticklabels (hiding minor tick labels is little tricky than I imagined). Updated image of result after fix.

@jreback I think it should be classified as a bug as the same as #7457 (#5897).

[sinhrks]

Modified to use try...except to avoid any issue caused by mpl.ticker

[jreback]

@sinhrks ping your unmerged issues that you think are ready

[sinhrks]

@jreback I think this one is ready.

[jreback]

thanks!