I am struggling with defining the x-axis range for an Excel plot. Please see description

I need the x-axis to have a bigger range of values that it plots the actual range against, if that makes sense? Most of the temperature readings were around 18.7C for the depth we tested and I want to show that as approximately a straight line but when I plot the data in excel, it is messy and has a slope to the right because it is plotting the range from say 18.6-18.7 and everything in between. I would like it to plot say, from 0-20C, which will leave the data points around 18.7C fairly close to each other, and should look more like a vertical line. Any ideas of how to do this? Here is a link to my file, I need the graph on the eight of the data to have an almost straight line vertically down, but the range is too specific at the moment, but I've been playing around with it and searching online, but cant find a solution. (https://onedrive.live.com/redir?resi...nt=file%2cxlsx) Thanks in advance

You sir are a gentleman and a scholar! Worked perfectly, didnt realize it gave me access to different formatting options. Had it on a scatter before per suggestion of another thread, but didnt go to the formatting settings to see the max and min range. Thanks again! :D