ENH: GH10485 'Frequency' label for Series.plot
More informative label for histogram.

Conversation in #10485 

Thanks! Please do:
- Fix the test cases under `pandas/tests/test_graphics.py`
- Add release note to `doc/source/whatsnew/v0.17.0.txt`
 @sinhrks Tests pass locally now. Is there a way to trigger another build on Travis?
 If you've updated this branch, please once rebase then push with force flag (`-f`). Travis test runs automatically.

https://github.com/pydata/pandas/wiki/Git-Workflows
 Ok, rebased. Build log shows this failure, which I believe is unrelated:

```
FAIL: test_metadata_immutable (pandas.tests.test_index.TestMultiIndex)
```
 Indeed, build failure was unrelated -- I just retriggered your tests
 @sinhrks build looks good now.
