PERF: compare freq strings (timeseries plotting perf)
See overview in #11084

@sinhrks You did think this is OK?
 Thanks for this. I think we should call `freq = Period._maybe_convert_freq(freq).freqstr` at the top of `extract_ordinals` for standardize the freq string. Because followings are valid constructions.

```
import pandas as pd

idx = pd.period_range('2011-01', periods=3, freq='A')
idx.asobject.values
# array([Period('2011', 'A-DEC'), Period('2012', 'A-DEC'), Period('2013', 'A-DEC')], dtype=object)

idx.freqstr
# 'A-DEC'

# freq = 'A'
pd.PeriodIndex(idx.asobject.values, freq='A')

# freq = <YearEnd: month=12>
pd.PeriodIndex(idx.asobject.values, freq=idx.freq)
```
 @jorisvandenbossche if you can update
 @sinhrks Thanks for the catch. It won't be needed for the plotting case I suppose, but indeed in general the example you give are valid. I should probably add some tests for these, as they were not caught by the test suite)
 @jorisvandenbossche which benchmark does this close?
 At least the plotting.plot_timeseries_period.time_plot_timeseries_period one (didn't test if it would improve for some of the others as well)
 this doesn't seem to fix that one?

and its using `date_range` not `period_range` (but same either way)
 Did you test with this branch?

```
import numpy as np
import pandas as pd

N = 2000
M = 5
df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975', periods=N))
```

With pandas 0.16.2:

```
In [18]: %timeit df.plot()
1 loops, best of 3: 196 ms per loop
```

With 0.17.0rc:

```
In [18]: %timeit df.plot()
1 loops, best of 3: 778 ms per loop
```

With this branch:

```
In [18]: %timeit df.plot()
1 loops, best of 3: 235 ms per loop
```

So it did not fully bring it back to previous performance, but at least not the big slow down.

It indeed uses `date_range` and not `period_range`, but the time series plotting machinery converts the datetimes internally to periods for plotting.
 ```
In [1]: N = 2000

In [2]: M = 5

In [3]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975', periods=N))

In [4]: %timeit df.plot()
1 loops, best of 3: 451 ms per loop

In [5]: pd.__version__
Out[5]: '0.17.0rc1+119.g9707214'
```

```
In [1]: pd.__version__
Out[1]: u'0.17.0rc1'

In [2]: N = 2000

In [3]: M = 5

In [4]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975', periods=N))

In [5]: %timeit df.plot()
1 loops, best of 3: 438 ms per loop
```

```
In [7]: import matplotlib

In [8]: matplotlib.__version__
Out[8]: '1.4.3'
```
 ```
In [6]: import matplotlib

In [7]: matplotlib.__version__
Out[7]: '1.5.0rc1'
```

```
In [1]: M = 5

In [2]: N = 2000

In [3]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975', periods=N))

In [4]: %timeit df.plot()
1 loops, best of 3: 499 ms per loop

In [5]: 2015-10-01 06:50:53.161 python[25071:8630508] setCanCycle: is deprecated.  Please use setCollectionBehavior instead
2015-10-01 06:50:53.163 python[25071:8630508] setCanCycle: is deprecated.  Please use setCollectionBehavior instead
2015-10-01 06:50:53.164 python[25071:8630508] setCanCycle: is deprecated.  Please use setCollectionBehavior instead
2015-10-01 06:50:53.166 python[25071:8630508] setCanCycle: is deprecated.  Please use setCollectionBehavior instead

In [5]: pd.__version__
Out[5]: '0.17.0rc1+119.g9707214'
```

@TomAugspurger can we get rid of this deprecated function?
 @jreback strange .. I just rebuild both versions (rc1 and this branch) and tested again:

```
In [1]: N = 2000

In [2]: M = 5

In [3]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975',
 periods=N))

In [4]: %timeit df.plot()
1 loops, best of 3: 734 ms per loop

In [5]: pd.__version__
Out[6]: '0.17.0rc1'
```

```
In [1]: N = 2000

In [2]: M = 5

In [3]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975',
 periods=N))

In [4]: %timeit df.plot()
1 loops, best of 3: 217 ms per loop

In [5]: pd.__version__
Out[5]: '0.17.0rc1+111.ga6379b6'
```

Did you test it with this branch or with current master? As I notice you have another commit in the version string
 ok cardinal sin of not rebuilding the extensions....i must have had an older version...

thanks!

```
In [1]: M = 5

In [2]: N = 2000

In [3]: df = pd.DataFrame(np.random.randn(N, M), index=pd.date_range('1/1/1975', periods=N))

In [4]: pd.__version__
Out[4]: '0.17.0rc1+118.g8ecec2d'

In [5]: %timeit df.plot()
1 loops, best of 3: 167 ms per loop
```
