title: Bokeh plot title 'str' object is not callable
body: <p>In Jupyter Notebooks I read in a dataframe and create several plots with Pandas / Bokeh.
  While creating one of the latter I get an error.
  Search for similar problems said, that there might be somewhere above in the script something like</p>
  <pre><code>plt.title = &quot;Title&quot;
  </code></pre>
  <p>which overwrites the method. But this is not the case for me. I have nothing similar in the code above -exept in the figure parameters. Here the Bokeh documentation describes to set a figure title like I used it.</p>
  <p>Using the part of the code that leads the the error in the complete notebook in a stand-alone script only does NOT lead to the error. So, also in my case the problem might have something to do with my code above. But maybe some of you has an idea when seeing this..(?)</p>
  <pre><code>import pandas as pd
  import numpy as np
  
  import matplotlib.pyplot as plt
  
  from bokeh.plotting import figure, show, output_notebook, ColumnDataSource
  from bokeh.io import output_notebook
  from bokeh.layouts import column, gridplot
  from bokeh.models import Label, Title
  from bokeh.models import Div
  
  data = df
  output_notebook()
  
  # Title of the overall plot
  abovetitle = (&quot;This should be the overall title of all graphs&quot;)
                 
  # GRAPH 1
  s1 = figure(width = 250, plot_height = 250, title=&quot;Graph 1&quot;, x_axis_label = &quot;axis title 1&quot;, y_axis_label = 'µs')
  s1.line(x, y, width=1, color=&quot;black&quot;, alpha=1, source = data)
  # s1.title.text = &quot;Title With Options&quot; # this is a instead-off 'title=' test, but does not solve the problem
  
  # GRAPH 2
  s2 = figure(width = 250, plot_height = 250, title=&quot;Graph 2&quot;, x_axis_label = &quot;axis title 2, y_axis_label = 'µs')
  s2.line(x, y, width=1, color=&quot;blue&quot;, alpha=1, source = data)
  #s2.title.text = &quot;Title With Options&quot; # this is a instead-off 'title=' test, but does not solve the problem
  
  # plot graphs:
  p = gridplot([[s1, s2]])  
  show(column(Div(text=abovetitle), p))
  </code></pre>
  <p>leads to the type error</p>
  <pre><code>---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-24-33e4828b986d&gt; in &lt;module&gt;
       31 # plot graphs:
       32 p = gridplot([[s1, s2]])
  ---&gt; 33 show(column(Div(text=title), p))
  
  TypeError: 'str' object is not callable
  </code></pre>
  <p>Recalling</p>
  <pre><code>import matplotlib.pyplot as plt
  </code></pre>
  <p>does not solve the problem. Hence, recalling</p>
  <pre><code>import pandas as pd
  import numpy as np
  
  import matplotlib.pyplot as plt
  
  from bokeh.plotting import figure, show, output_notebook, ColumnDataSource
  from bokeh.io import output_notebook
  from bokeh.layouts import column, gridplot
  from bokeh.models import Label, Title
  from bokeh.models import Div
  </code></pre>
  <p>solves the problem. Any further idea what might cause this error?</p>
  <p>In the mean time I got a very useful hint: In one of the prior cells I accidentially used a Bokeh API function name as variable name and overwrote the function. If someone faces a comparable problem have a look at your variable naming. Maybe there happend the same accident... ;-)</p>
  <pre><code>#############################
  # Define column names of XData binary part
  header = [&quot;Col1&quot;,&quot;Col2&quot;,&quot;Col3&quot;] 
  
  # Split XData in single, space separated columns 
  x_df = selected_df.XData.str.split(' ', expand=True)
  x_df.drop(0, inplace=True, axis=1)
  x_df.columns = header
  #print(x_df)
  
  # Binary XData to integer
  for column in x_df:         # DONT DO THAT!!!!! THIS OVERWRITES BOKEH API FUNCTION. EG. USE `col` INSTEAD OF `column`
      x_df[column] = x_df[column].apply(int, base=16)     # DONT DO THAT!!!!! THIS OVERWRITES BOKEH API FUNCTION. EG. USE `col` INSTEAD OF `column`
  </code></pre>