I was also looking for same thing but i couldn't found. Unlike excel we can not move data point on the graph manually. May be there is some other way but what i know is that we can reduce the number of data points from the graph to make it visible.
In the pivot table properties, goto dimension tab and mark the "Suppress When Value Is Null" in Setting for Selected Dimension... or may be i am gettign you wrong.. Can you please explain which multi line you are talking about?