Hubble Constant Question

Can anyone help me?
I've been given a report to write about calculating the hubble constant
using standard rods. I've caluculated the hubble constant for 15 galaxies
from the hubble deep field. The instructions i have were to plot these
values against the recession velocity for each galaxy and observe a trend.
Done that. But what causes the trend (increasing hubble constant with
velocity) and how do i remove it?