Excel 2007 : Change chart source data in XL2007

Thanks in advance for any replies.
I have charts in a rather complex spreadsheet comprising 12 tabs in total, but the chart itself is quite simple. It originated in XL2003 but simply charts values on the vertical axis against dates on the horizontal axis. Having now come to the end of 2009, I want to continue tracking the values into 2010. The source data is on another worksheet which extends as far into the future as a I want. I have successfully updated the source data by editing in the "select data source" tab which now reflects the correct data source up to 30/6/2010. However, the chart itself refuses to change at all. it continues to chart only up 31/12/2009. XL2003 allowed formatting of the horizontal axis itself but 2007 doesn't seem to offer that function. I have no idea where to go from here. Any suggestions would be most appreciated.

Thank you Teylyn, A bit unintuitive but I eventually found the command you refer to. Problem fixed. Happy New Year and thanks so much