can't make a chart from data imported from txt file
 

well.. I thought I have done whole project... I thought :D
here is my problem:

I import data to txt files, they look like this:

2006-08-25 14:45:08 89,1 g
2006-08-25 14:45:15 50 g
between each column is a Tab

When I import those files I cannot make a chart from 3rd column.. unleas I
double click on each value...
I don't have to do it with e.g. 50 but must with 89,1
I know that it is probably the problem with comma..
How can I make a chart without double clicking