title: Initialize y-axis to be centered at 0
body: <p>I'm generating some line plots that show percent error results. It would be nice if the y-axis was centered at 0 when the plots first load. I know I can manually set the bounds for the y-axis, but it would be tedious to have to set them manually for each figure. </p>
  
  <p>Is there a way to set the figures to center at 0 along the y-axis?</p>
  
  <p>Here's some code that may provide additional details - </p>
  
  <pre><code>from bokeh.plotting import figure, output_file, show
  from bokeh.io import gridplot, hplot
  
  # prepare some data
  x = [1, 2, 3, 4, 5]
  y1 = [-10, -9, 23, 4, -6]
  y2 = [-15, -4, 26, 32, -45]
  y3 = [-42, -20, -13, -34, -59]
  y4 = [-23, -34, -32, -43, -53]
  
  # output to static HTML file
  output_file("lines.html", title="line plot example")
  
  # create a new plot with a title and axis labels
  p = figure(title="simple line example", x_axis_label='x', y_axis_label='y')
  p.line(x, y1, line_width=2)
  p.line(x, y2, line_width=2)
  
  p2 = figure(title="simple line example", x_axis_label='x', y_axis_label='y')
  p2.line(x, y3, line_width=2)
  p2.line(x, y4, line_width=2)
  
  p = hplot(p, p2)
  show(p)
  </code></pre>
  
  <p>This generates two plots. The one on the right shows the problem I'm running into. Since all of the values are negative, the y-axis bounds are narrowed to approximately -10 to -60. I would like that chart to be bound at -60 to 60, so that it's aligned at 0.</p>
  
  <p>Update: 
  I ended up just defining a function that will return the absolute max of the y values. Then, I am doing the following to set the limits:</p>
  
  <pre><code>axisLimit = getAxisRange(list1, list2) #get absolute max from the two lists
  p.y_range = Range1d(start=-1*axisLimit, end=axisLimit)
  </code></pre>