Reading Tables, Charts and Graphs

Elements of Visual Displays and Line Graph Analysis

Visual displays present structured data to allow rapid comprehension of relationships and values. A graph's title identifies the main subject of the visual display, giving context to the overall information presented. Along the borders, axis labels identify the variables plotted and include their units when applicable, ensuring that measurements are understood accurately. The independent variable is traditionally plotted along the horizontal x-axis, whereas the dependent variable is traditionally plotted along the vertical y-axis. When a display includes multiple sets of data, a legend or key explains the symbols, patterns, or colors used to distinguish multiple data series.

An axis scale shows how numerical values correspond to positions along an axis, but its intervals are not always uniform. Reading axis scale intervals carefully prevents errors caused by non-unit grid line increments, such as grid lines that advance by multiples rather than single units. Furthermore, readers must remain alert to graph design choices: a truncated or broken axis on a graph can visually exaggerate differences between values, making small changes appear substantially larger than they actually are.

A line graph is commonly used to show how a numerical value changes over time or another ordered numerical variable, even when observations are recorded only at discrete points. Individual points on a line graph represent coordinate pairs of values corresponding to the two axes. Line segments connect plotted values and show the change between them: an upward segment has a higher ending value, a downward segment has a lower ending value, and a horizontal segment has equal endpoint values; they do not establish what happened between observations. The steepness of a line segment represents the rate of change between two data points, showing how rapidly a quantity rises or falls.

When tracking multiple subjects simultaneously, a multiple-line graph uses two or more lines to compare trends for different groups across the same x-axis values. Interpreting line graphs also involves evaluating values beyond or between recorded data points. Estimating an unknown value between two known plotted points on a line graph is called interpolation. Conversely, projecting a value beyond the range of known data points on a line graph is called extrapolation. To calculate net change over an interval on a line graph, subtract the initial value from the final value.