A fraction is a single value. At is neither a graph nor a curve.
I am not sure what situation you are talking about; usually such curves should be smooth.
If you know the data should be smooth, then yes. But if there are sharp resonances you could miss such things altogether.
It is not true to say that you should automatically draw a smooth line. It depends on what you are attempting to do. A smooth line may be a good indicator of a trend but such a line is useless if you want to find out whether or not there is any periodicity in the data. For the latter you must join the data points and look for periodic patterns.
You need to consider all of the key players. You should figure out how to make the start as smooth as possible.