title: Histogram multiple columns, percentage per columns same plot
body: <p>I have this dataframe(df_histo):</p>
  <pre class="lang-py prettyprint-override"><code>ID   SA   WE   EE
  1   NaN  320  23.4 
  2   234  2.3  NaN
  ..  ..  ..   ..
  345 1.2  NaN  234
  </code></pre>
  <p>I can plot a correct histogram if I use <code>density=True</code>,</p>
  <p><code>buckets = [0,250,500,1000,1500,2000,2500,3000,3500,4000,4500,5000]</code></p>
  <p><code>plt.hist(df_histo, bins=buckets, label=dfltabv_histo.columns, density=True)</code></p>
  <p>however I want to visualize the data(1 histogram) in total percentages for each country. That is, what percentage each bins represents in that country</p>
  <p>the closest attempt has been this(with individual columns it works):</p>
  <p><code>plt.hist(dfltabv_histo, bins=buckets, label=dfltabv_histo.columns, weights=np.ones(len(dfltabv_histo)) / len(dfltabv_histo))</code></p>
  <p><code>plt.gca().yaxis.set_major_formatter(PercentFormatter(1))</code></p>
  <p><code>ValueError: weights should have the same shape as x</code></p>