automatic inversion of x axis by pandas.plot(...)
X axis was inverted automatically and unexpectedly when plotting a series of data against another series of data using pandas. 
My example code blow creates three plots, only some, not all, of which shows inverted x axis. I think this behavior is very confusing for users even if there was some rationale behind it. IMHO, automatic inversion of x axis is unnecessary because a user can use invert_xaxis() in case one wants to invert it. On [stackoverflow](http://stackoverflow.com/questions/29908874/x-axis-inverted-unexpectedly-by-pandas-plot), a workaround was suggested, but no direct solution.

```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
df2 = pd.DataFrame(np.random.randn(10, 3), columns=["a", "b", "c"])
df3 = df2*1.1

df3.rename(columns={"a": "a*1.1", "b": "b*1.1", "c": "c*1.1"}, inplace=True)
df23 = df2.join(df3)

fig, ax_list = plt.subplots(1,3)

ax=ax_list[0]
df23[["a", "a*1.1"]].plot(ax=ax, x="a")
ax.axis('equal')
ax.set_title("(x,y)=(a,a*1.1)")
print ax.get_xlim()  ## Added for clarity

ax=ax_list[1]
df23[["b", "b*1.1"]].plot(ax=ax, x="b")
ax.axis('equal')
ax.set_title("(x,y)=(b,b*1.1)")
print ax.get_xlim()  ## Added for clarity  

ax=ax_list[2]
df23[["c", "c*1.1"]].plot(ax=ax, x="c")
ax.axis('equal')
ax.set_title("(x,y)=(c,c*1.1)")
print ax.get_xlim()  ## Added for clarity
```
