title: Plotly - Histogram bins size to weeks
body: <p>I'm trying to plot a histogram with date data using plotly. I would like to plot it with bin sizes corresponding to weeks, and that doesn't seem to work. I searched for documentation about it but didn't find anything.</p>
  
  <p>Here is the code I have. I tried (line 5): 'D7' and 'W1'. That doesn't work (plotly seems not to recognize argument, and set it to one bin per day). What's strange is that 'M1', 'M3' etc... seem to work</p>
  
  <pre><code>fig = go.Figure(data=[go.Histogram(x=df.col, 
                                     xbins=dict(
                                         start='2018-01-01',
                                         end='2018-12-31',
                                         size='D7'), 
                                     autobinx=False)])
  fig.update_layout(
      title=go.layout.Title(
          text="title",
          xref="paper",
          x=0.5
      ),
      xaxis_title_text='xaxis title',
      yaxis_title_text='yaxis title'
  )
  
  
  fig.show()
  </code></pre>
  
  <p>Would someone have any information about this problem ?
  Thanks</p>