Page:Graphic methods for presenting facts (1914).djvu/92

 the data according to the scale chosen. Thus the figure 222 for the year 1840 would be indicated as a dot on the vertical line for 1840 slightly above the place where the horizontal scale line for 200 crosses the chart. After the dots for decade years have been placed on the vertical lines, the dots are joined with a heavy line and a curve appears.

The curve of Fig. 68 shows the changes from decade to decade much better than the bars of Fig. 67. It can be seen at once from the curve that the greatest gain in any decade was between 1870 and 1880 when the increase was three and a half divisions on the vertical scale. The bars in Fig. 67 have no horizontal scale to measure by and the comparison between census years is accordingly more difficult.

When plotting any curve the vertical scale should, if possible, be chosen so that the zero of the scale will appear on the chart. Otherwise, the reader may assume the bottom of the chart to be zero and so be grossly misled. Zero should always be indicated by a broad line much wider than the ordinary co-ordinate lines used for the background of the chart.

In Fig. 68, it will be noticed that figures are given at the top of the chart to represent the value for each point plotted on the curve. The use of figures at the top of the chart in this manner is very desirable. The figures are in plain sight, so that anyone desiring to know the value of any point on the curve can look above the point to get the actual figure wanted, without having to read from the scale at the left-hand edge and then estimate roughly the value for any point which happens to fall in a space between two horizontal lines of the scale. Reading from the figures at the top of the chart permits any desired figure to be obtained more rapidly and much more accurately. In addition to this, the figures are recorded in such manner that they may be quoted for use elsewhere by anyone who may wish to make use of the data in a speech or in a written article in which the chart itself cannot be used.

It would be a desirable thing if in all curve charts the figures for the horizontal scale were placed at the bottom of the chart rather than at the top. Many illustrations in this book, taken from publications of excellent standing, show dates (such as years, months, etc.) at the top of the chart. If the horizontal scale were always placed at the bottom, the standard arrangement would be a convenience to the reader and would give the additional advantage that the top of the