I have just opened a spreadsheet that I use every month (saved as .xls but
opened in Excel 2007) and then gone to the chart to update the data to show
Feb's figures. But when I click with my mouse on the chart, nothing happens.
It won't select anything. I tried doing a new chart but even that won't
respond to my mouse.

However, if I copy the data into a new spreadsheet and generate a chart, it
responds just like it's meant to. What am I doing wrong in the existing
spreadsheet?