b. Curve or Line Charts. This chart provides a way to present
cumulative or noncumulative events that occur over a period of time
(time series). Since the person viewing the chart can readily follow a
line, the line creates the impression of moving through time and
magnitude with the data. For example, you could use this type of chart
to show the number of hours spent performing operator maintenance each
month. You can design a line chart to show more than one variable, an
average, or desired rate line.
When designing a curve or line chart, use the same process you used for
a column chart when selecting the scale, title, and chart construction.
A completed curve or line chart presents a grid system for plotting the
data with the vertical and horizontal line drawn.
When plotting data on a curve or line chart, you plot single points
only. The point is the intersection of the data and scale lines when
you connect the points using the method desired for the chart. After
you have plotted and connected the points, you ink and letter the chart
using the same process used for a column chart.
There are several types of curve or line charts: broken curve, smooth
curve, and step curve.
(1) Broken curve or line chart. This type of chart presents
noncumulative quantities at a given time. This means the values plotted
represent the total at a specific time. Even though a straight line
connects the points on the chart, you cannot assign any intermediate
values from this line. The line shows direction or trend, and it does
not assign any values. When using this type of chart, you must take the
data from the same day each month. Otherwise, the chart will not
present an accurate picture of the totals.
For example, figure 124 shows a broken curve chart that presents data
showing that 50 people graduated from the onthejob training program in
July, and 60 people graduated from the onthejob training program in
August. Someplace between the plotted points for the two months the
line crosses the 55 quantity line. However, that does not mean you can
conclude that at that point in time 55 people graduated from the
program.
(2) Cumulative (smooth) curve or line chart. When using this type
of chart to present data, the reader can assign intermediate values from
the running, smooth curve. This type of curve or line is an effective
way of presenting the growth or decline of an activity. When you use
this chart with a projected curve or line, plotting the actual data
shows trend,
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