Help with average

I have two things for you to try. The first is to look at that duration field and there is probably a setting to force a certain unit of measure. By that I mean instead of letting it default to smart units force it to display in minutes or in hours.  

If that does not solve the issue then I suggest you make a formula which converts the duration in either the number of numeric minutes or the number of numeric hours and then plot your chart using a traditional numeric field.