Error in graph with lambda about streamz

[CJ-Wright]

Also it seems that we could use a more robust assert than just checking the file's existence.

[jrmlhermitte]

having same problem:

In [1]: from streamz import Stream

In [2]: s = Stream()

In [3]: s2 = s.map(lambda x : x + 1)

In [4]: s.visualize()
Error: Unknown HTML element <lambda> on line 1 
in label of node map; <lambda>
---------------------------------------------------------------------------
CalledProcessError                        Traceback (most recent call last)

but this works:

In [1]: from streamz import Stream

In [2]: s =Stream()

In [3]: def inc(x):
   ...:     return x+1
   ...: 

In [4]: s2 =s.map(inc)

In [5]: s.visualize()
Out[5]: <IPython.core.display.Image object>

[jrmlhermitte]

Should be closed with #81. @CJ-Wright let us know otherwise. thanks for the input!