One of the disadvantages of the otherwise excellent graphic method of presenting facts is, that while the diagrams give to the eye a very definite impression, it is oftentimes difficult to reproduce this impression in language. In other words, diagrams as such can not be quoted. On the scales in which they are usually presented, exact measurements are impracti c'able. It follows therefore, that the use of graphics is to emphasize a general impression, and in order 'that this impression may be as concrete as possible it is a useful rule, too often neglected, that the facts dis played by line or solid should at the same time be given in terms of figures.
22. The appeal to the eye.—The purpose of graphi cal presentations is to make an appeal to the eye, and it follows at once that this appeal must be simple and direct. The diagram should not impose too great a burden on the vision, or too fine a discrimination be tween the points which are depicted. When the dia gram is made by means of lines drawn to a scale, there is oftentimes a temptation to put too much on the diagram, with the result that there are often sev eral interesecting lines. Without the use of color, and this is in most cases not available, it is very diffi cult to draw, at the best, more than two or three lines which can be clearly distinguished one from another. Hence the best usage in charts of this character calls for not more than' two or three lines, and this, of course, means a decided limitation upon the amount of material which can be displayed in a given space.
Where the eye is called upon to judge relative size as, for instance, when the exports of different years or the exports of different countries are compared, the most effective presentation is the line diagram. The
use of squares, circles or other surfaces, and especially flat representation of solid bodies, is to be avoided.
Perhaps the greatest sinners against this rule of sim plicity are those who address themselves to the least intelligent audience, and who in school books and in magazines of popular circulation, represent, for in stance, the exports of different countries by pictures of sheep, ingots of gold, bales of cotton or hogsheads of wine, of different sizes. The foregoing illustration will make this point clear, and speaks so plainly for itself that additional comment is not necessary.
23. most frequent way in which graphical statements are made is by the use of the line, or as it is sometimes technically called, "curve," in which the height of the curve from the base meas ures the magnitude of the phenomenon in question, while the distance from the left-hand side of the page represents the period of time to which the facts relate.
The rule of simplicity makes it advisable that all distances should be measured by the eye from the same base. If we disregard this rule, and try to represent different elements which enter into the problem by superimposing one on the other, with the thought that in this way we obtain an aggregate, the result is dis astrous. Only two lines in the entire chart are clear, the first above the base and the line which represents the aggregate. The other lines cannot easily be read by the eye, and if we want to observe their tendency they must be taken out of the chart and redrawn so that for each of them there is a uniform base from which to measure the height. Where the lines are drawn from the same base, it is sometimes expedient