title: Plotly plot axis line thickness not behaving [Python]
body: <p>When I try to increase the x-axis thickness, plotly overlays a new axis line (of the desired thickness), instead of just increasing the thickness of the existing axis line. Am I doing something wrong? For example if I edit the layout property ‘xaxis’:{‘linewidth’:3}, it produces a new line under the xaxis (above the xlabels) which has width of 3, but it looks bad obviously. What can I do about this? I’ve tried by editing ‘zerolinewidth’:3 as well but this does nothing. </p>
  
  <p>Here's an example code with the issue reproduced - if we remove the xaxis property 'linewdith':3 in the layout dict then the plot is default. I just want control over the axis linewidth.</p>
  
  <pre><code># Example axis problem
  import plotly.offline as pyo
  from plotly.graph_objs import *
  from plotly import tools
  import plotly.plotly as py
  pyo.offline.init_notebook_mode()
  
  xdata = list(range(6))
  ydata = [0, .2, .4, .6, .8, 1]
  
  trace1 = {'type':'scatter',
       'x':xdata,
       'y':ydata,
       'mode':'markers+lines'}
  
  layout = {'title':'some plot',
       'xaxis':{'title':'x-axis',
               'linewidth':3},
       'yaxis':{'title':'y-axis'}}
  
  data = Data([trace1])
  
  fig = Figure(data=data, layout=layout)
  
  pyo.iplot(fig)
  </code></pre>