9. Situations have been known where two lines are ill with HCD simultaneously. At different stages though.
10. Some lines seem to be much less prone to HCD, or even HCD-free possibly. Need to confirm that latter tentative assertion. Duration: at least for a good long while, if not permanently.
11. Shapes of the HCD phenomenon vary somewhat.
(i) The classic shape is a concave-upwards parabola, whose axis of symmetry is vertical, and which has two local maxima at the endpoints. (ii) Another common early-stage-only shape is on the rhs, past the overall maximum, and is right-downwards sloping. The depth is very shallow and the rh endpoint may not be a exactly local maximum, but more like a point of inflection, or close to one. The distinguishing feature of this type is that at the point where the curve is furthest away from the most appropriate test chord, the value of the derivative dy/dx is not very close to zero, but is always at least slightly negative.
(iii) Recently seen for the first time is a subtype related to ii. Would welcome suggestions as to what it should be called. Here is a picture taken just a few hours after its first appearance:
In the section to the right of x=58, the value of day/dx is fairly close to zero and in the first section, 58-63 at least, the derivative is not negative. (Noise is ignored here and these comments everywhere are after an imagined smoothing post-processing to not only get rid of noise, but to remove all stalactites too. So comments involving the derivative should take this imagined filtering/post-processing into account.)
I could do with names for these subtypes’ shapes’ names. Any volunteer contributors?