Diagram over temp when temp is below zero

Hi. I'm tring to do a diagram over temp and time.
Outside temp is often below zero. But when i do a line diagram I get all values for outside tempeture to zero even those which are ex 6 plus.
I have lowered the axel to minus 15 but the line is still all on zero?

Any Ideas what to do?
Cant excel handle negitve numbers?

Br
Tobias