formatting: scale_x_datetime should apply date-time formatting to the breaks

[alshan]

t = pd.to_datetime("2021-01-01")
(ggplot() 
  + geom_vline(xintercept=t)
  + scale_x_datetime(format="%d %b\'%y"))

works as expected:

Scale with 'breaks' doesn't work:

(ggplot() 
  + geom_vline(xintercept=ut)
  + scale_x_datetime(format="%d %b\'%y", breaks=[t]))

(ggplot() 
  + geom_vline(xintercept=ut)
  + scale_x_datetime(format="{%d %b\'%y}", breaks=[t]))

Wrong number pattern: %d %b'%y

(ggplot() 
  + geom_vline(xintercept=t)
  + scale_x_datetime(breaks=[t]))

After converting pandas Timestamp to "UNIX time" It doesn't work either:

# UNIX time (delta from UNIX epoch in ms)
ut = (pd.to_datetime("2021-01-01") - pd.Timestamp("1970-01-01")) // pd.Timedelta("1ms")

(ggplot() 
  + geom_vline(xintercept=ut)
  + scale_x_datetime(breaks=[ut]))

[OLarionova-HORIS]

Applying date-time formatting to breaks works now:
#421 - date-time format support was added to StringFormat.