Very tall Y axis for high BMI on trisomy 21 charts

[mbarton]

Porting across rcpch/digital-growth-charts-server#220 to this repo as it seems to be an issue with the chart rendering rather than the data returned from the API.

Birth Date: 08/05/2006
Measurement Date: 01/11/2022
BMI: 36.3 kg/m²

Storybook story to reproduce in the mbarton/trisomy21-bmi-rendering-issue branch: 6aa9181

[mbarton]

I think the issue is with the bmiSDSData returned from getDomainsAndData:

digital-growth-charts-react-component-library/src/functions/getDomainsAndData.ts

Line 523 in b6331d2

function getDomainsAndData(

For the 3.33 SDS line (the rightmost tallest one in the screenshot) the values ramp up quickly and then end up null:

[mbarton]

Ah I see those are the actual values from the reference set:

digital-growth-charts-react-component-library/src/chartdata/trisomy21_bmi_male_sds_data.ts

Line 7712 in b6331d2

"y": 4204.5954

I think I will need @eatyourpeas input here on how that reference was calculated. There's also rcpch/digital-growth-charts-server#217 raised already for the nulls

[eatyourpeas]

These values were calculated from our chart-coordinates endpoint and are possibly correct because of the way the maths works at higher values. The BMI curves in trisomy I find much less useful clinically for this reason. @statist7 would better be able to comment whether this is an actual error or whether this is more an issue for rendering where we simply apply an arbitrary upper limit to the visible value of y.

[statist7]

For particular configurations of L, M, S and z, BMI heads off to infinity. The relevant formula is Centile = M(1+LSz)^(1/L) so if 1+LSz = 0 then there is a problem.

This happens when L is negative (which it nearly always is with BMI) and z is sufficiently large and positive. As @eatyourpeas says, perhaps test for and trap large BMI values.