What version of FileMaker are you using?
If using Filemaker 12, make sure to specfiy a time data type in the Y-Axis options.
Here's a quick demo file: https://dl.dropboxusercontent.com/u/78737945/TimeChartDemo.fmp12
We have not yet moved the database to FileMaker 12, but thank you for showing me that. I did try FileMaker 12 and could not figure it out. We are currently on FileMaker 11. If anyone knows a way to do it, or if it is not possible, let me know...
Charting was a new feature with FileMaker 11. FileMaker 12 added more capabilities and this was one of the new capabilities.
In the future, please let us know what version of FIlemaker you are using when you post a question. It can save us wasting our time and yours with a solution that does not work with your version.