title: Numerical formatting of label ticks for charts in bokeh
body: <p>Consider the following scatter plot example. </p>
  
  <pre><code>import bokeh
  from bokeh.charts import Scatter, output_file, show
  from bokeh.models import HoverTool
  
  p = Scatter(df, x='colA', y='colB', title="Foo", color="navy",
              xlabel="A", ylabel="B", tools="hover")
  </code></pre>
  
  <p>I would like to change the formatting of the tick labels for one axis:</p>
  
  <p>When I try with:</p>
  
  <pre><code>p.xaxis[0].formatter = NumeralTickFormatter(format="0.0%")
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>Chart object has no attribute xaxis
  </code></pre>
  
  <p>Why? How can I access the figure of the chart?</p>