Any non-vertical straight line (you'd be in a fine pickle if your weight trend line were vertical, wouldn't you?) can be expressed in the form:
Y = mX + b
where m is the slope, giving the change in the Y axis value for each unit change along the X axis, and b is the intercept, the point at which the line crosses the Y axis when X is zero.
To find m and b for the line that best fits a collection of data points D1, D2, ... Dn we calculate:
Since we're only interested in the rate of change, we only need the slope, m, which gives the daily rate of change in the line that best fits the moving average trend curve. From the slope, the average weight change per week is just seven times the daily change,
and the average daily calorie deficit (if negative) or excess (if positive) is: