Inconsistent default date ticks for data at different frequencies
When the data indexed with datetime are plotted via DataFrame.plot(), the default date ticks behave inconsistently depending on the frequency of the data index.

Here's a [short notebook](https://gist.github.com/marcinbielecki/9a8c1e3f7900a4761202769aecf746e0) that reproduces the issue. The data at daily, weekly and annual frequencies are plotted with ticks aligning to "round" years, in this example multiples of 5 (5n). However, data at monthly and quarterly frequencies are plotted with ticks aligning to 5n-1.
