How to send axhline to back of Matplotlib's barplot
<p>I have the following code. It uses <a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.plot.html" rel="noreferrer">Matplotlib wrapper in Pandas</a>:</p>

<pre><code>import pandas as pd
import io
import matplotlib
import matplotlib.pyplot as plt

test=u"""Cell,Value
Bcells,0.056304
DendriticCells,0.3155388
Macrophages,0.131430
"""

tableau10=[ "#17BECF", "#BCBD22", "#7F7F7F", ]

toplot_df = pd.read_csv(io.StringIO(test))
toplot_df.set_index('Cell',inplace=True)
xlabels =  toplot_df.index.values
barplot   = toplot_df.plot(kind="bar", figsize=(17,17), \
                          color = tableau10, \
                          width=0.7,\
                          fontsize = 30,\
                          legend=False,
                          ylim = (0,0.5),
                          subplots=False)

ax = plt.gca()
ax.set_xticklabels(xlabels, rotation=30, ha='right')

# How to to make this to the back
plt.axhline(y=0.1, linewidth=1, color='r',zorder=1)
plt.xlabel("")
plt.ylabel("Score", fontsize=30, fontweight="bold")
</code></pre>

<p>And it makes this figure:</p>

<p><a href="https://i.stack.imgur.com/obNpq.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/obNpq.jpg" alt="enter image description here"></a></p>

<p>As stated there. How can I make the <code>axhline</code> to the background?
I tried <code>zorder</code> but won't work.</p>