Args for column can not be used

[conan-goldsmith]

https://github.com/edenhill/librdkafka/blob/107a5f9ed7040cae56ca2993f4508e86194983b3/tests/tools/stats/graph.py#L49

Currently, the graph.py code checks if the skips arg is empty, if it is then it asserts the cols argument must also be empty. This means the only way to use the cols argument is by explicitly providing an empty set for the skips argument. This is either a typo on the error line or an IF missing on the argument check for the cols line to be verified before assertion.

[edenhill]

Oh, you're right, that assert should be in the else: part of that if-statement!

[edenhill]

Feel free to submit a PR, Conan