title: Get selected data contained within box select tool in Bokeh
body: <p>If I have a scatter plot in bokeh and I've enabled the Box Select Tool, suppose I select a few points with the Box Select Tool. How can I access the (x,y) position location information of the points that I've selected?</p>
  
  <pre><code>%matplotlib inline
  import numpy as np
  from random import choice
  from string import ascii_lowercase
  
  from bokeh.models.tools import *
  from bokeh.plotting import *
  
  output_notebook()
  
  
  TOOLS="pan,wheel_zoom,reset,hover,poly_select,box_select"
  p = figure(title = "My chart", tools=TOOLS)
  p.xaxis.axis_label = 'X'
  p.yaxis.axis_label = 'Y'
  
  source = ColumnDataSource(
      data=dict(
          xvals=list(range(0, 10)),
          yvals=list(np.random.normal(0, 1, 10)),
          letters = [choice(ascii_lowercase) for _ in range(10)]
      )
  )
  p.scatter("xvals", "yvals",source=source,fill_alpha=0.2, size=5)
  
  select_tool = p.select(dict(type=BoxSelectTool))[0]
  
  show(p)
  
  # How can I know which points are contained in the Box Select Tool?
  </code></pre>
  
  <p>I can't call the "callback" attribute and the "dimensions" attribute just returns a list ["width", "height"]. If I can just get the dimensions and the location of the Selected Box, I can figure out which points are in my dataset from there.</p>