In the Chart Data table, when I enter a single ":" or ":" followed by a number 
and apply it to the chart, OO gives me a "Invalid Number" pop-up. However, when 
I enter a number followed by a ":", OO instead applies a completely different, 
arbitrary value to the chart in its place and changes the data field in the 
table to that value.  This is also true for the character "/".  To reproduce 
this:

1.) Select the "Insert Chart" option, either from the toolbar or the "Insert" 
pull-down menu.
2.) Right-click the chart and select the "Chart Data" option.
3.) In the data field of Row 1, Column 1, type exactly the characters within 
the quotations "9:".
4.) Click the "Apply to Chart" button and observe the chart.

When I did this, I got the value 0.375.  You can also do this for "/", just 
replace the ":" with "/".