In June of 2011 my A1c value was 6.2, and John decided it was time to declare me a Type II diabetic. I immediately changed my diet, and you can see the effects -- 3 months later my A1c value was back down to 5.0 %, which is the top of the normal range for that metric.
I was lucky; John is an enlightened physician. And he compared values backwards across my various labs, at least one step back. But I wish he'd started with the A1c alot sooner. A1c measures how much sugar is in your blood averaged over a few weeks. This is important because even if your fasting blood glucose levels are good, after a meal your blood sugar can spike way up if you are pre-diabetic. And those peaks damage things I value, like nerve cells! Even though my fasting BG value was lower in June 2011 than it had been, my A1c value was higher. After changing my diet my A1c values have been excellent (between 5 and 5.4), even though my fasting glucose values tend to still be high. So I could have been in trouble on the A1c scale much sooner, if I had been tested. I could swear that I've seen a graph of mortality as a function of A1c that suggested values above 5 were really, really bad, but I've been unable to find it. There are lots of studies that show increasing A1c increases all sorts of bad outcomes, but they mostly bin values together so you can't see the effect of a value of 5.5 vs. 5. Certainly, going above 6 % is bad.
So, making a graph makes the data trend much clearer than simply seeing the lab results one value at a time. Your doctor won't do this. You can.
I'm not going to describe the changes I made to my diet to get this result here; that's for another day. However, I want to connect this post to the Adaptive Management theme of the blog. In 1995 I made a major shift in my diet to very low fat Vegan following the recommendations of Dr. Dean Ornish, the McDougalls, and others. I had swallowed the dietary fat = death paradigm of the time. And it worked, at least for a while. I lost weight. My blood lipids improved. But 5 years later my blood lipids were worse than before, and 10 years after that I was diagnosed diabetic. I had to change the underlying hypotheses about the relationship between diet and health, at least for me.
*Names may be changed to protect the innocent!