Excel 2007 : graphing a continuous data series/assuming zeros

Hello,
I have been working on a project during which I have had to keep track of the number of bird present at a feeder at any given point in time over the course of an hour and I had to do this multiple times a day for a week. Now that I have collected my data I am trying to graph the information but am struggling to get excel to do what I want it to do (and dont even know if its possible_.

I have been trying to graph time passed on the x-axis and number of birds present on the right. The problem is that the lowest number of birds I have recorded is 1 and excel assumes that is the lowest it needs to go and connects the 1s with a straight line. Is there any way that I can create a graph with a horizontal axis representing a 60 minute period of time where any point without a value is assumed to be 0? If that doesnt make sense, I am essentially trying to make a line graph that goes down to 0, but not having to enter 0 for that point in time.

Also, how does excel deal with duplicates? Since I did this multiple days, there is some overlap in data and it would be nice if just sum two data entries matching the same time period.

Thanks in advance, sorry if that is confusing