In addition if you extend the graph to present day it becomes extremely obvious that what had been an upwards trend for some 30 years reversed decisively by 2003. If you exclude the large Shipman anomaly of 2002 and the cockle pickers of 2003 it seems like the trend probably reversed (or at least plateaued) in 2001, 4 years after the law came in. I'm sure it wasn't the only factor, but "This law was introduced and then 4 years later a year-on-year rise in murders suddenly became a year-on-year fall" doesn't help your case much, considering you'd expect there to be a little while before the impact is felt.
Well the obvious point here is that a year by year fall doesn't necessarily show anything in the long run. As the chart shows, the short term trend was often decisively downwards yet the long term trend was upwards; from '68 to '08, the average homicide rate was 52% higher, while from '97 to '08 it was 15% higher. It could, admittedly, be reversing, yet lets look at long trends in US states/the US proper at the same time:
Washington D.C vs the US
-The rate of homicide averages 73% higher during the gun ban, yet the U.S rate of homicide averages 11% lower.
Chicago vs the US
-The rate of homicide in Chicago after the gun ban averages 17% lower while the rate of homicide in the U.S averages 25% lower, which seemingly shows that it worked but...
-The percentage of murders committed using handguns actually INCREASES dramatically when the gun is put into place, clearly showing that the gun ban made no difference to the decline of crime.
Inversely, let's see what happened in Florida when right to carry came into effect...
-Floridian homicide rates average 36% lower after the introduction of right to carry, whereas U.S homicide rates average 15% lower, a significant difference.
Whatever the case in the short term, it's pretty obvious that American homicide rates have been declining steadily for the past 50 years whereas British homicide rates have been increasing, and the states with the highest decrease in homicide rates are those which are trending towards looser gun control laws.
What is state above is that the states which were required to enforce the brady bill saw the same decline in homicides as states which already had the restrictions. It's up to interpretation, and specifically is not proof that the law had no effect, the same report says:
"On the plus side, there is strong evidence that the law undermined gun-running operations that were buying large numbers of guns in southern states and transporting them north for resale, he said."
So there's "strong evidence" that the Brady Bill reduced the flow of illegal guns from states with previously poor gun control to those with stonger gun control.
Yet (A) The states which introduced right to carry legislation were the ones that saw the largest decline in crime afterwards in comparison and (B) California is neither referenced in the article provided nor qualifies as a "northern state" (for the record, California's restrictions were almost exactly the same as those imposed by the Brady law, and its decline in crime without any additional increases in restrictions was 18% whereas in the rest of the nation it was 7%).