The caption read like "down about 329 g a week — roughly 460 g a day then,
320 g a day now". Subtract the two amounts and you get 140 g, not 329. The
arithmetic behind it was self-consistent, but the sentence was not, and a
caption a reader can disprove by subtracting its own numbers is wrong whatever
the code was doing.
The rate was slope x 7, while the line only covers the complete days in the
window. Today is never fitted, being unfinished, so a 7-day window leaves at
most five days and any marked day takes another — in this case about three.
The rate was therefore stretched well past the days it was measured from, and
the two endpoints, which were not, could never agree with it.
It quotes the change between the two ends now, which is the one figure a reader
can check: "down about 140 g — from roughly 460 g a day to 320 g." The change
is derived from the rounded ends rather than from the slope, so the subtraction
works exactly rather than to within the rounding.
The weekly rate goes rather than being repaired. It cannot reconcile on a short
window, and it only ever meant anything where the fit spanned a week or more —
which is not something to leave as a trap for whichever window the reader
happens to have picked.
The check that let this through asserted the sentence contained certain
phrases, not that its numbers agreed with each other. There is now one that
parses all three figures back out and asserts the move is exactly the
difference of the ends, across each window length; it fails against the old
wording, which is the only evidence worth having that it would have caught this.
"Up about 40 g a week" is a rate with nothing to anchor it: it says the line
slopes without saying where it sits. The sentence now names both ends of the
fit — "roughly 280 g a day then, 400 g a day now" — which is the reading anyone
actually wants from a growth chart.
When the fit is not trustworthy it quotes the average instead, and says the
day-to-day variation is larger than any trend. That difference is the point.
The average is a measurement and survives the noise; the ends of the line are
the line's own output, and quoting them on a fit nobody should read would dress
a guess up as a reading. Everything rounds to 10 g for the same reason — "287 g
a day" would be false precision from four noisy points.
The wording moves into a pure foodTrendSentence() so those rules can be
checked, which is worth doing precisely because they are judgement rather than
arithmetic: the checks now pin that a clear climb gives both figures, a flat run
gives the average and no endpoints, a see-saw gives neither, and nothing is ever
quoted to the gram.
The daily grams bars bounce around enough to hide a steady climb, so they
cannot answer the question you actually have about a growing puppy: is he
eating more than he was? A least-squares fit through them can.
Two kinds of day stay out of the fit. Today is half-eaten, and including it
would pull the line down every morning and let it drift back up as meals go in
— a line that tracks the clock rather than the dog. A day marked "not counted"
has a hatch rather than a figure, and fitting a zero there would invent a dip.
The line is drawn only across the days it was fitted on, so it never implies it
knows about the ones it skipped.
The caption is the part that needed the care. A straight line through seven
noisy points will always have a slope, and announcing it as a fact is the same
mistake the walking goal made. So a direction is named only when the fitted
climb is larger than the scatter of the days around it, and only when it clears
5 g a week and a twentieth of a typical day; otherwise it says the variation is
larger than any trend, which over a short window is usually the truth.
It names its window too — "over the last 14 days" — because the 7/14/30 picker
already drove this (weeklyData builds the array the fit runs on) but nothing on
screen said so, and the line moves too little between windows to show it. When
there are fewer than four complete days it now says why there is no line rather
than leaving bars with nothing through them.
Meals can be logged without an amount, so the note counts them: a day can read
low because he ate little or because nobody typed the number, and the chart
should not let those look the same.
The checks cover the refusals rather than the arithmetic — a see-saw is not
reported as a trend, a slope under the scatter is not either, three days will
not fit, a marked day does not shift the line, and each window length reaches
the fit intact.
Alexander Heldt