Why doesn't my resonance tube graph pass through zero?

[Miki]

I am havin a bit of trouble w/ a graf here... its about resonance waves in resonance tubes and i have a straight line graph when i plot the values of length of the wave vs. 1/frequency. the problem is, i don't no what this line/slope is meant to represent? and it doesn't run through zero, wot type of error is dat calld?
cheerz miki

 

[Doc Al]

You may find this discussion helpful: https://www.physicsforums.com/showthread.php?t=75691

 

[Miki]

Wow tanx so much, dats like exactly wot we had 2 du tanx a lot!