Departure from this rule should never be made except where there is a special reason for so doing.
Procedures
1. WHEN NECESSARY. The zero line or other base of comparison should never be omitted when the interest is in relative amount of change between points on the same curve.
2. WHEN NOT NECESSARY. When the interest of the reader is in the absolute amount of change rather than in the relative amount of change, it may be safe to omit the principal point of reference and the accompanying horizontal line.
3. OMISSION SHOULD BE INDICATED. When the zero value or other principal point of reference is omitted the fact should be clearly indicated in a manner that will attract notice.
Note: Since It Is generolly token for granted that the base line is the zero line, it is not sufficient merely to show the bose line as a light ruling instead of the customary heavy ruling.
4. EFFECTIVE METHODS OF INDICATING the omission of the zero point and line:
(a) A wavy line across the bottom of the grid, (bl A straight line waved at each end.
STANDARDS FOR TIME SERIES CHARTS
RANGE AND SPACING OF AMOUNT SCALE
Principl*!
1. Since the omount scole has a controlling effect on the movement oi the curves it is highly important that a scale be selected which will result in a true picture of the facts.
2. The amount scale should be divided in a manner that will facilitate accurote reading of the curve values.