The progress chart of my weight log showed a measured weight curve that was not a function of the time, looping and backtracking. This is due to the automatic smoothing in the ProgressChart graph, which uses cubic-splines. See attached screenshot 1.
This behaviour is fixed by un-protecting the cell, right-clicking on the weight line, "Chart Type"/"Smooth Line Properties" and checking "B-spline" instead of "Cubic splines": B-splines are an approximation of a real-value function from R to R, hence respect the "no-backtrack on X-axis", while cubic-splines are an of a parametrized curve from R to R^2, not guaranteeing the "no-backtrack on X-axis".
See attached screenshot 2.
Dataset causing problem was having a whole 15 days of missing data due to holidays:
85.0
Empty
85.0
Empty
84.0
84.2
Empty
84.1
83.8
83.3
82.8
82.4
81.5
81.8
82.3
82.1
Then 16 Empty entries
83.1
81.6
82.0
81.9
81.7
81.4
Additional precision: B-splines of order > 1 do not pass exactly by the points, on the countrary to the cubic spline approximation. Even so, I'd rather have a non-smoothed measured weight curve than a time-travelling curve, since the other curve gives the smoothed trend anyways.