title: How to align trace text to center in a stacked horizontal bar chart
body: <p>I have a stacked horizontal bar and I want text defined for each trace to be placed at the center of the corresponding bar. I can't find an attribute that sets this without using anotations, but I'd like to use "text" for each trace and just be able to align it.</p>
  
  <p>I'm using Plotly 3.4.1 with Jupyter (Plotly offline). 
  Couldn't find any documentation about how to do this except trying to do this with annotations, which look like a more suitable solution if I want to pinpoint an explicit coordinate. What I want is a much simpler (something like "align": "center"), but couldn't find any attribute for this under go.Bar</p>
  
  <p>Just want the "80", "20" to appear at the center instead of aligned to the right</p>
  
  <pre><code>from plotly.offline import iplot, plot, init_notebook_mode
  import plotly.graph_objs as go
  
  def getStackedSentimentHbar():
      trace0 = go.Bar(
          y=["A","B"],
          x=[20,80],
          orientation = 'h',
          text=["20","80"],
          textposition="inside",
          hoverinfo = "none",
      )
      trace1 = go.Bar(
          y=["A","B"],
          x=[80,20],
          orientation = 'h',
          text=["80","20"],
          textposition="inside",
          hoverinfo = "none",
      )
      data = [trace0,trace1]
      layout = go.Layout(
          barmode='stack',
          showlegend=False,
          xaxis=dict(
              showgrid=False,
              zeroline=False,
              showline=False,
              ticks='',
              showticklabels=False
          ),
          yaxis=dict(
              showgrid=False,
              zeroline=False,
              showline=False,
              ticks='',
              showticklabels=True
          ),
          margin = dict(
              l = 200, 
              r = 50, 
              b = 50, 
              t = 50, 
              pad = 10
          ),
          font=dict(
              family='Heebo', 
              size=18, 
              color='#000000'
          )
      )
      fig = go.Figure(data=data, layout=layout)
      return fig
  
  init_notebook_mode()
  fig = getStackedSentimentHbar()
  iplot(fig)
  </code></pre>