Chart for PV problems

Hey everyone, I'm having an issue with a chart in Excel 2007. I have a column of PVs of bonds. Now due the type of bond I'm using (Premium, Discount, Par) these PV values either decrease, increase, or stay the same throughout.

I'm not sure how relevant the above information is but here is my question. When my Present Values stay the same on the whole problem my chart goes haywire. My chart tracks PV over time, but instead of showing a straight line, it goes all over the place...

Can anyone help?

I'm new to the site so if its possible for me to upload my spreadsheet please let me know. Thanks!

Thanks AndyPope. I have attached my spreadsheet.

As you can see, when B9 and B12 are equal, H8:H27 are also equal. The chart should show a straight line, but it doesn't.

Btw, its for a college class.

for some reason it worked when both are 10% but if you tried other equal amounts, it jumped around. Thankfully, I figured out the problem though, the axis was set for auto min and max which for some reason set it at (999,1000). I went in and made it manual for (0,1000) and it works fine now.