I wish they'd make sure each and every health visitor understood how to use the percentile graphs.
In my life and profession I've come across great health visitors. Highly knowledgeable, insightful and progressive. I've actually interviewed the pioneering guy who created the percentiles graph and improved public health as a consequence. So interesting and an honor. Didn't want to talk about hv misinterpretation though! And last week I interviewed a brilliant independent hv in the east of England, who was great, really inspiring and the end of the call devolved into a discussion about my sons up and coming (routine) op and how young children heal...
But in my personal life and role as a mother, I've not been lucky enough to have the same experiences. Sigh.
It seems that any baby not on the 50 percentile is in terrible danger which the mother must avert immediately, even if they're tracking the same line steadily...
I had experience of bonkers advice - for gaining and reducing weight, all with the same baby!
DS was born massive and it was suspected undiagnosed gestational diabetes. Rest of family all tall & very slim so the consultant said we should expect a plunging line until he found his natural weight not artifially boosted by the extra sugars. So we were discharged after DS blood sugars had stabilized, with instructions to keep an eye out, extra doctors checks, but not to panic if he lost weight but was feeding well and otherwise thriving.
I get that movement across percentiles is worth looking at, but not just one but two hv ignored the consultants diagnosis and advice, for no discernible reason they preferred their own random interpretations. What other profession would be able to disagree with a diagnosis completely with no evidence or rationale for it? Then tried to persuade me that he didn't need doc monitoring, just... A diet! Or force feeding, depending on the hv.
First one said I must limit my 5 week olds milk and refuse to feed on demand, as my baby was 'too fat' and I must be feeding him too much! Breast feeding was met with pursed lips as it was more important to measure & control his milk intake 
Avoided her for the next appointment and got the polar opposite advice, that he was falling through percentiles and though it was healthy when he was falling to 50%, but when he broke through the 50th line - to 48%, it was suddenly a sign he was terribly unwell. So the reasoning was that I must be starving him, my supply must be drying up and not good enough quality, and so I must wake him up hourly to force feed him, again bottle considered better...
Both hv refused to believe in the gestational diabetes, and preferred their own made up stories of what might be happening. Grrrr. Very confusingly neither thought that DS needed to see a doctor (GP or paed), in spite of the doomsday predictions.
At this point I stopped with hv appointments, and by 12 weeks he was gliding happily along the 25% and continued to do so.
Thought that was it, but at his 2.5yr check the hv did try and restart the panic as he was too underweight...
Apparently the height & weight percentiles should mirror each other and if not, there's something horribly wrong.
So 98% height, 25% weight steady for the last 2 years is cause for alarm for some reason! I did break at this point and explain that this wasn't the case at all, otherwise every single person would have the same height to weight ratio and there would only be one body type... Which clearly isn't the case! She wasn't convinced.
Thank goodness I've not had to go again.