How should I write very long lines of code?

Where to look for help in future

Most syntax problems like this are dealt with in PEP 8. For the answer to this question, you can refer to the section "Code Layout".

Preferred way : Use (), {} & []

From PEP-8:

The preferred way of wrapping long lines is by using Python's implied line continuation inside parentheses, brackets and braces. If necessary, you can add an extra pair of parentheses around an expression...

This means your example would like like this:

url= ('http://chart.apis.google.com/chart?chxl=1:|0|10|100|1,000|10,000|' +
      '100,000|1,000,000&chxp=1,0&chxr=0,0,' +
      max(freq) + 
      '300|1,0,3&...chco=A2C180&chds=0,300&chd=t:')

The alternate way: Use \

From PEP-8:

...but sometimes using a backslash looks better. Make sure to indent the continued line appropriately.

url = 'http://chart.apis.google.com/chart?chxl=1:|0|10|100|1,000|10,000|' + \
      '100,000|1,000,000&chxp=1,0&chxr=0,0,' + \ 
       max(freq) + \
      '300|1,0,3&...chco=A2C180&chds=0,300&chd=t:'

Avoiding concatenation

String formatting

In this case, we only have a single thing that we would like to be changed in the URL: max(freq). In order to efficiently insert this into a new string, we can use the format method with numerical or named arguments:

url = "http://...{0}.../".format(max(freq))
url = "http://...{max_freq}.../".format(max_freq=max(freq))

Python combines two strings literal together, so

>>> s = "abc" "def"
>>> s
'abcdef'

but that wouldn't work if they are on two lines because Python doesn't know that the next line is part of the command. To solve that you can use backslash or brackets.

>>> s = ("hello, world"
"!")
>>> s
'hello, world!'

and you wouldn't need + to attach the strings together. You will still need it for adding nonliterals like max(freq), as explained in String Literal Concatenation. This is marginally more efficient, but more importantly clearer and enables commenting parts of a string as shown in the Python documentation linked.